Preheat the oven to 375°F (190°C).
Cook the lasagna noodles according to package instructions until al dente. Drain and set aside.
In a large skillet, heat olive oil over medium heat. Add diced onion and garlic, sautéing until translucent.
Add zucchini, yellow squash, and red bell pepper to the skillet. Cook for about 5-7 minutes, until softened. Stir in spinach until wilted. Season with Italian seasoning, salt, and pepper. Remove from heat.
In a separate pot, melt butter over medium heat. Stir in flour, whisking for 2-3 minutes until golden. Gradually add milk, continuing to whisk until the mixture thickens. Season with salt and pepper.
In a mixing bowl, combine ricotta cheese, half of the Parmesan cheese, salt, and pepper.
To assemble the lasagna, spread a thin layer of the béchamel sauce on the bottom of the baking dish. Layer 4 lasagna noodles, half of the ricotta mixture, half of the sautéed vegetables, and a third of the mozzarella cheese. Repeat the process, ending with noodles and béchamel sauce. Top with remaining mozzarella and Parmesan cheese.
Cover the baking dish with aluminum foil (to prevent sticking, spray the foil with cooking spray). Bake for 25 minutes.
Remove the foil and bake for an additional 15 to 20 minutes, until bubbly and golden on top.
Let it cool for 10 minutes before slicing and serving.