Preheat your oven to 375°F (190°C). Bring a large pot of salted water to a boil and cook the lasagna noodles according to the package instructions. Drain and set aside.
In a large skillet, heat the olive oil over medium heat. Add the diced onion and minced garlic, and sauté until the onion becomes translucent and fragrant.
Add the diced zucchini, red bell pepper, and yellow bell pepper to the skillet. Cook for about 5 minutes, until the vegetables begin to soften.
Stir in the sliced mushrooms and cook for another 5 minutes, until they release their juices and start to brown.
Add the spinach leaves to the skillet and cook until wilted, stirring occasionally.
Pour in the diced tomatoes and tomato sauce, and add the dried oregano, dried basil, salt, black pepper, and red pepper flakes (if using). Stir well to combine and let the sauce simmer for about 10 minutes to allow the flavors to meld together.
In a separate bowl, combine the ricotta cheese, grated mozzarella cheese, and grated Parmesan cheese. Mix well until all the cheeses are fully incorporated.
To assemble the lasagna, spread a thin layer of the vegetable sauce on the bottom of a 9x13-inch baking dish. Place 3 lasagna noodles on top of the sauce, slightly overlapping them. Spread half of the ricotta cheese mixture over the noodles, followed by a layer of the vegetable sauce. Repeat this process with another layer of noodles, ricotta cheese, and vegetable sauce.
For the final layer, place the remaining 3 lasagna noodles on top, followed by a generous amount of vegetable sauce. Sprinkle the grated mozzarella cheese on top, ensuring that the noodles are fully covered.
Cover the baking dish with aluminum foil and bake in the preheated oven for 25 minutes. Then, remove the foil and bake for an additional 10 minutes, until the cheese is melted and bubbly.
Once the lasagna is done baking, remove it from the oven and let it cool for a few minutes. Garnish with fresh basil leaves before serving.
Enjoy this delicious and wholesome vegetable lasagna with your loved ones!