I’ve been making these plum oatmeal bars every late summer when local plums are at their peak. They’re a simple, rustic dessert (or hearty breakfast bar) with a buttery oat base, jammy plum filling, and a crunchy crumble top — all in one pan. They’re ideal for brunch, packing in a lunchbox, or serving at a potluck when you want something fruit-forward but not fussy.

Why you’ll love this dish

These bars hit a lot of home-run boxes: seasonal fruit, pantry-friendly oats and flour, and a make-ahead format that improves after a chill in the fridge. They’re not overly sweet, so the natural tartness of plums shines through. The crumble gives a satisfying texture contrast — chewy base, jammy fruit, crisp topping.

“Perfect balance: sweet, tart, and oat-forward — like a portable plum pie.” — a regular tester

When to make them: late-summer plum season, brunches, school bake sales, or as a simple dessert for weeknights. They’re also a budget-friendly way to use an abundance of fruit.

The cooking process explained

Before you dive in, here’s what happens in the pan:

Mix oats, flour, sugars, spices, and melted butter to make a crumbly dough.
Press half into a pan and par-bake so the base sets and won’t go soggy.
Toss chopped plums with cornstarch to thicken their juices.
Spread fruit over the warm crust, sprinkle the remaining crumb mixture on top, and bake until golden and bubbly.
Cool completely, then chill so the bars slice cleanly.

This short overview helps you pace the work: par-bake the crust first, prep fruit while it’s baking, then assemble and finish-bake.

Gather these items

2 cups old fashioned rolled oats (not instant) — for texture; quick oats will make a softer crumb.
1 cup all-purpose flour — helps bind the oats; substitute 1:1 gluten-free flour for GF.
1/2 cup granulated white sugar — for brightness.
1/2 cup brown sugar, packed — adds molasses notes and chew.
1 teaspoon baking powder — lightens the crumb slightly.
1 teaspoon ground cinnamon — warm spice; optional.
1/2 teaspoon kosher salt — balances sweetness.
1 cup melted unsalted butter — use melted coconut oil for a dairy-free version (changes flavor).
1 teaspoon vanilla extract — rounds flavor.
6 plums, pitted and cut into ~1-inch pieces — ripe but still slightly firm is best.
1 tablespoon cornstarch — thickens fruit juices as it bakes. (Arrowroot works for GF.)

Step-by-step instructions

Preheat and prep. Heat the oven to 375°F (190°C). Line a 9 x 9-inch square pan with parchment paper, leaving an overhang on two sides. Lightly mist the parchment with nonstick spray.
Mix dry ingredients. In a large bowl whisk together oats, flour, granulated sugar, brown sugar, baking powder, cinnamon, and salt until even.
Add butter and vanilla. Pour in the melted butter and vanilla. Stir with a sturdy spoon until the mixture is crumbly and holds together when pressed between fingers.
Par-bake the base. Reserve about half the oat mixture for the topping. Press the other half firmly and evenly into the bottom of the prepared pan. Bake 10–12 minutes until edges just begin to turn golden.
Prepare plums. While the crust bakes, chop plums into ~1-inch pieces. Toss the fruit with 1 tablespoon cornstarch until evenly coated.
Assemble. Spread the cornstarch-coated plums over the warm par-baked crust in an even layer. Crumble the reserved oat mixture over the plums.
Final bake. Return pan to oven and bake 30–40 minutes. The topping should be light golden and juices should bubble slightly at the edges.
Cool and chill. Remove from oven and cool completely on a rack. Refrigerate at least 1 hour to firm up for clean slicing.
Slice and serve. Use the parchment overhang to lift the slab from the pan. Cut into squares with a sharp knife. Serve chilled or at room temperature.

What you’ll need (ingredients recap)

2 cups old fashioned rolled oats (not instant)
1 cup all-purpose flour
1/2 cup granulated white sugar
1/2 cup brown sugar, packed
1 teaspoon baking powder
1 teaspoon ground cinnamon
1/2 teaspoon kosher salt
1 cup melted unsalted butter
1 teaspoon vanilla extract
6 plums, pitted and cut into ~1-inch pieces
1 tablespoon cornstarch

(See substitutions above for gluten-free or dairy-free swaps.)

Serving suggestions

Best ways to enjoy it: Warm with a scoop of vanilla ice cream, or chilled with a cup of strong coffee for breakfast.
For brunch: Serve alongside yogurt and granola or a cheese plate for contrast.
Party plating: Cut into small squares and garnish with a light dusting of powdered sugar or a lemon zest sprinkle to brighten the plum flavor.
Kids’ lunchbox: Wrap individual squares in parchment — they travel well if kept cool.

Storage and reheating tips

Refrigerator: Store in an airtight container for up to 5 days. Chilling keeps the bars firm and makes tidy slices.
Freezer: Wrap individual squares tightly in plastic wrap and place in a freezer bag for up to 3 months. Thaw in the fridge overnight.
Reheat: Warm a square in the microwave for 10–15 seconds or 6–8 minutes at 350°F (175°C) in a small ovenproof dish to revive the crumble texture.
Safety note: Because these contain fresh fruit, keep at or below 40°F (4°C) if storing and discard after recommended times.

Pro chef tips

Par-bake the base: That 10–12 minute par-bake prevents a soggy bottom and gives structure.
Cornstarch is key: Tossing fruit in cornstarch avoids runny bars; use arrowroot for a glossy, GF finish.
Press firmly: Pack the base into the pan with a flat-bottomed measuring cup for an even, compact crust.
Cutting trick: Chill fully. Use a long, sharp knife warmed under hot water and wiped dry between cuts for cleaner edges.
Sweetness balance: If your plums are very sweet, reduce granulated sugar by 2–3 tablespoons.
Ripeness matters: Very ripe plums release more juice; slightly firm ripe fruit holds shape better.

Creative twists

Fruit swaps: Substitute peaches, nectarines, or a mix of berries and plums. Adjust cornstarch if berries are used (they release more juice).
Nutty crunch: Fold 1/2 cup chopped toasted almonds or pecans into the topping mixture.
Spice changes: Add 1/4 teaspoon ground ginger or 1/8 teaspoon cardamom for an aromatic twist.
Vegan version: Use melted coconut oil instead of butter and ensure brown sugar is vegan.
Lower-sugar option: Cut both sugars by 25–30% and add a drizzle of honey or maple after baking if needed.

Helpful answers

Q: Can I use frozen plums?
A: Yes. Thaw, drain excess liquid, and toss with cornstarch before using. Be aware frozen fruit tends to be softer and may require an extra tablespoon of cornstarch.

Q: Can I make these ahead?
A: Absolutely. Bake, cool, and refrigerate for up to 5 days. They also freeze well for up to 3 months — thaw in the fridge overnight.

Q: How do I get neat slices?
A: Chill at least 1 hour, then use a sharp knife warmed under hot water and wiped dry between cuts. Parchment overhang lets you lift the slab cleanly from the pan.

Q: Can I make this gluten-free?
A: Yes. Use certified gluten-free oats and swap the all-purpose flour for a cup-for-cup gluten-free flour blend that contains xanthan gum.

Q: Why par-bake the crust?
A: Par-baking firms the base so it won’t become soggy from the fruit juices, resulting in a distinct bottom layer rather than a mushy mess.

Conclusion

If you want a dependable, seasonal bar that’s equal parts breakfast and dessert, these plum oatmeal bars are a great choice. For more inspiration on similar fruit-crumble bars and variations, try the Plum Crumble Breakfast Bars recipe at Cuisine Fiend and explore the Apple and Plum Crumble Bars at Entirely Elizabeth for a different spin on fruit and oat textures.