In a large skillet, heat 2 tablespoons of olive oil over medium heat. Add 1 cup of diced eggplant and 1 cup of zucchini, sautéing for about 5 minutes until they're soft. Then, toss in 1 chopped red bell pepper, cooking for an additional 3 minutes. Finally, stir in 2 cups of fresh spinach and 2 minced garlic cloves until the spinach wilts, about another minute.

In a microwave-safe bowl, warm 2 cups of milk until it's steaming but not boiling, about 1-2 minutes. In a separate saucepan, melt 4 tablespoons of unsalted butter over medium heat. Whisk in 1/4 cup of all-purpose flour, cooking until it turns golden, roughly 2 minutes. Gradually pour in the warmed milk, whisking constantly for about 5-7 minutes until the mixture thickens. Stir in 1/2 cup of grated Parmesan cheese, seasoning with salt and pepper to taste.

Preheat your oven to 350°F (175°C). Spread a thin layer of marinara sauce on the bottom of a 9x13-inch casserole dish. Lay down 3-4 no-boil lasagna noodles, then top them with half of the béchamel sauce, followed by half of the sautéed vegetable mix, and 1 cup of shredded mozzarella cheese. Repeat these layers, finishing with the remaining béchamel and a generous layer of mozzarella on top.

Cover the assembled Easy Vegetarian Lasagna with aluminum foil and bake it in the preheated oven for 45 minutes. After this, carefully remove the foil and increase the temperature to 400°F (204°C). Bake for an additional 10-15 minutes until the cheese is bubbly and golden brown.

Once baked, remove the lasagna from the oven and let it rest for 15-45 minutes before slicing.