Gather all your ingredients. Wash and chop the vegetables as specified. Finely chop the onion and mince the garlic; slice the zucchini and yellow squash thinly, and dice the red bell pepper.

In a large skillet, heat the olive oil over medium heat. Once hot, add the chopped onion and garlic, sautéing until tender and fragrant, about 3-4 minutes.

Add the zucchini, yellow squash, mushrooms, and red bell pepper. Cook for another 5-7 minutes until the vegetables are slightly softened.

Stir in the fresh spinach and season with salt, pepper, and Italian seasoning. Cook until the spinach wilts, about 1-2 minutes. Remove from heat and let cool slightly.

In a separate bowl, combine the ricotta cheese, egg, and grated Parmesan until creamy and smooth.

Preheat your oven to 375°F (190°C). Start layering the lasagna in a baking dish. Spread a thin layer of marinara sauce on the bottom.

Add three cooked lasagna noodles, then half of the sautéed vegetable mixture, followed by more marinara sauce and half of the ricotta mixture. Repeat the layering.

Place the final layer of noodles on top. Spread the remaining marinara sauce over the noodles and cover with shredded mozzarella cheese.

Cover the baking dish with aluminum foil and bake for 25 minutes. Remove foil and bake for another 15-20 minutes until cheese is bubbling and golden.

Let the lasagna rest for 10-15 minutes before cutting and serving.