Prepare the Sauce

1. Sauté the Aromatics: In a large saucepan, heat the olive oil over medium heat. Add the chopped onion and cook for about 3-4 minutes, until softened. Add the minced garlic and sauté for another 30 seconds.
2. Make the Sauce: Stir in the crushed tomatoes, tomato paste, oregano, basil, and red pepper flakes (if using). Let the sauce simmer for 15-20 minutes, stirring occasionally, until it thickens slightly. Season with salt and pepper to taste. Set aside.

Prepare the Vegetables

1. Sauté the Veggies: In a separate pan, heat the olive oil over medium heat. Add the diced zucchini, bell pepper, and mushrooms. Cook for 5-7 minutes, until the vegetables are tender. Add the spinach and cook for an additional 2-3 minutes, until wilted. Remove from heat and set aside.

Cook the Noodles

1. Boil the Noodles: Bring a large pot of salted water to a boil. Cook the lasagna noodles according to package instructions, or skip this step if using no-boil noodles. Drain and set aside.

Assemble the Lasagna

1. Layer the Lasagna: Preheat the oven to 375°F (190°C). In a large baking dish, spread a thin layer of tomato sauce on the bottom. Place a layer of lasagna noodles over the sauce. Next, add a layer of sautéed vegetables, followed by a generous dollop of ricotta cheese. Sprinkle a handful of mozzarella and Parmesan cheese on top. Repeat this process until all the ingredients are used, ending with a final layer of noodles and sauce. Top with a generous layer of mozzarella and Parmesan.
2. Bake: Cover the lasagna with aluminum foil and bake for 30 minutes. Remove the foil and bake for an additional 10-15 minutes, until the cheese is bubbly and golden. Let it rest for 10 minutes before slicing and serving.