I first made this slow cooker Mongolian beef on a rainy weeknight when I wanted takeout flavors without the delivery wait. The sauce is glossy, sweet-and-savory, and it practically falls apart into tender ribbons of flank steak after a few hours in the crock. It’s an easy weeknight winner when you want big flavor with minimal hands-on time.

Reasons to try it

This version of Mongolian beef is tailor-made for busy evenings: it’s low-effort, uses affordable flank steak, and delivers a restaurant-style sauce without standing over the stove. It’s great for meal prep, family dinners, and feeding a crowd because you can double the recipe and keep the sauce consistent. If you prefer a quicker finish, the cornstarch slurry thickens the sauce right in the slow cooker so there’s no separate pan to clean.

“Comforting, sticky, and addictive — just like takeout, but I made it in my slow cooker.” — Home Cook Review

If you like other hands-off beef meals, you might also enjoy this slow cooker roast beef for a different but equally simple dinner option.

Step-by-step overview

Make the sauce by combining soy, brown sugar, water, garlic, and ginger in the slow cooker.
Add thinly sliced flank steak and cook slowly until tender (low 4–5 hours or high 2–3 hours).
Thicken the cooking liquid with a cornstarch slurry and finish on high for about 30 minutes.
Serve hot over rice or steamed veggies and garnish with green onions.

This quick outline helps you scan the process before jumping into ingredients and detailed steps.

Gather these items

1 lb flank steak, sliced thinly (slice across the grain for tenderness)
1/2 cup soy sauce (use low-sodium to control salt)
1/4 cup brown sugar (adjust to taste)
1/4 cup water
2 cloves garlic, minced
1 tsp grated fresh ginger
1/4 cup green onions, sliced (scallions)
2 tbsp cornstarch
2 tbsp vegetable oil (optional, for searing)
Cooked rice or steamed veggies, for serving

Notes: If you need gluten-free, swap soy sauce for tamari or coconut aminos. For a lighter sugar option, reduce brown sugar to 3 tablespoons and add a splash of rice vinegar if you want more acidity.

Step-by-step instructions

Whisk the sauce: In the slow cooker, combine the soy sauce, brown sugar, water, minced garlic, and grated ginger. Stir until the sugar begins to dissolve.
Prep the beef: Slice the flank steak thinly against the grain. Thin slices shorten cooking time and improve tenderness.
Add the steak: Add the sliced beef to the slow cooker and toss to coat evenly in the sauce.
Cook low and slow: Cover and cook on low for 4–5 hours, or set to high for 2–3 hours, until the beef is tender.
Thicken the sauce: Mix the cornstarch with 2–3 tablespoons of cold water to form a smooth slurry. Stir the slurry into the slow cooker. Cook on high for an additional 25–30 minutes until the sauce has thickened and become glossy.
Finish and serve: Stir in the sliced green onions, taste and adjust seasoning, then serve over hot cooked rice or steamed vegetables.

Tip: If you prefer a slightly crisped finish, you can remove the cooked beef and quickly sear it in a hot skillet with a little oil for 30–60 seconds per side before returning to the sauce.

Best ways to enjoy it

Serve over steamed jasmine or brown rice for a classic pairing.
Spoon over cauliflower rice or roasted broccoli for a low-carb option.
Plate with stir-fried snow peas, carrots, and bell peppers for color and crunch.
Garnish with sesame seeds and extra sliced green onions. For a spicy kick, drizzle in sriracha or sprinkle red pepper flakes.

Pair the dish with a simple cucumber salad or steamed bok choy dressed with a splash of rice vinegar to cut through the sweetness.

Storage and reheating tips

Refrigerate: Cool leftover beef to room temperature and transfer to an airtight container within 2 hours. Store in the refrigerator for up to 3–4 days.
Reheat: Warm gently on the stovetop over low-medium heat, stirring occasionally. Add a splash of water if the sauce has thickened too much. Microwave reheating works fine for single portions—cover and heat in 30–45 second bursts.
Freeze: Freeze cooled portions in freezer-safe containers for up to 3 months. Thaw overnight in the refrigerator before reheating.

Food safety note: Make sure leftovers reach an internal serving temperature of 165°F (74°C) when reheating.

Pro chef tips

Slice against the grain: Cut flank steak across the grain into thin strips to maximize tenderness.
Sear for depth: Quick-searing slices of beef in vegetable oil before adding to the slow cooker adds savory depth, though it’s optional if you want true hands-off prep.
Control salt: Use low-sodium soy sauce if you want to reduce sodium — you can always add a pinch more at the end.
Adjust sweetness and heat: Add more brown sugar for sweeter sauce or a dash of chili paste for heat. Taste after the sauce thickens and tweak.
Don’t over-stir: Stir gently after adding the slurry—vigorous stirring can break up the beef too much.

Creative twists

Broccoli beef finish: Add broccoli florets in the last 30 minutes so they stay bright and crisp-tender.
Sesame-ginger upgrade: Stir in 1 tsp toasted sesame oil and 1 tbsp toasted sesame seeds before serving for a nutty note.
Protein swaps: Swap flank steak for thinly sliced sirloin, skirt steak, or even boneless chicken thighs (adjust cook times). For a vegetarian option, use firm tofu—add in the last hour to avoid over-softening. You might also like this high-protein alternative slow cooker garlic butter beef bites for a different flavor profile.

Your questions answered

Q: Can I use a different cut of beef?
A: Yes. Sirloin or skirt steak work well. Flank is affordable and flavorful, but slice thin and cook until tender. Tougher cuts like chuck can be used but will need longer cooking and may shred rather than slice.

Q: How long does it take to thicken the sauce?
A: The cornstarch slurry will usually thicken the sauce within 20–30 minutes on high in the slow cooker. If it’s still thin, remove the lid for the last 10 minutes to reduce liquid, or thicken briefly on the stovetop.

Q: Can I prepare this ahead of time?
A: Absolutely. Assemble in the slow cooker insert, refrigerate (covered) for up to 24 hours, then cook from chilled—add an extra 20–30 minutes on high if needed. If frozen, thaw before cooking for even results.

Q: Is this recipe gluten-free?
A: Swap the soy sauce for tamari or coconut aminos to make it gluten-free. Confirm your brown sugar and other ingredients are certified gluten-free if necessary.

Q: Can I double the recipe?
A: Yes—use a larger slow cooker to allow room for even heat circulation. Cooking times remain similar, but check for tenderness as larger volumes can sometimes need slightly longer.

Conclusion

If you want another slow-cooked version with familiar flavors, these two reliable guides are great references: check out Slow Cooker Mongolian Beef – Dinner at the Zoo for another take on the method, and Easy Slow Cooker Mongolian Beef Recipe – The Chunky Chef for extra tips and variations. Both resources pair well with the techniques here and can inspire additional tweaks to make this recipe your own.