Preheat your oven to 375°F (190°C).

Cook the lasagna noodles according to package directions until al dente. Drain and lay them flat to prevent sticking.

Heat olive oil in a skillet over medium heat. Add sliced mushrooms and sauté until they are soft and any released liquid has mostly evaporated (about 6–8 minutes).

Add fresh spinach, ½ tsp salt, and ½ tsp black pepper. Cook until the spinach is fully wilted. Remove from heat.

In a saucepan, melt butter over medium heat. Stir in flour and cook for about 1 minute until it looks bubbly and slightly golden.

Gradually whisk in the milk, keeping the mixture smooth. Cook until the sauce thickens and coats the back of a spoon (3–5 minutes).

Stir in grated Parmesan and garlic powder. Remove the sauce from heat. Taste and adjust salt if needed.

Spread a thin layer of sauce in the bottom of a 9×13-inch baking dish. Lay 3 noodles side by side.

Top with half the mushroom-spinach mixture, half the ricotta (dollop and gently spread), and one-third of the shredded mozzarella.

Repeat: 3 noodles, remaining mushroom-spinach, remaining ricotta, and another third of mozzarella.

Finish with the last 3 noodles and the remaining sauce spread evenly on top. Sprinkle the remaining mozzarella over the top layer of sauce.

Cover the dish tightly with foil and bake covered for 25 minutes.

Remove foil and bake another 15–20 minutes until the top is golden and bubbly.

Let the lasagna rest for 10 minutes before slicing and serving.