Preheat your oven to 400°F (200°C). Chop zucchini, eggplant, and red bell peppers into bite-sized pieces. Toss them in a large bowl with olive oil, salt, and pepper until evenly coated. Spread the vegetables on a baking sheet and roast for 20–25 minutes, turning halfway, until they are tender and slightly caramelized.

In a large pot over medium heat, warm a tablespoon of olive oil. Add chopped onion and sauté until translucent, about 5 minutes. Stir in minced garlic and cook for an additional minute until fragrant. Pour in the crushed tomatoes, tomato paste, oregano, basil, salt, and pepper, then let it simmer for 15–20 minutes, allowing the flavors to meld beautifully.

In a mixing bowl, combine ricotta cheese, fresh spinach, and a beaten egg. Add half of the grated Parmesan cheese, along with salt and pepper to taste. Mix until smooth and well blended, ensuring that the spinach is evenly distributed throughout the ricotta for a creamy filling in your vegetarian lasagna.

Bring a large pot of salted water to a boil and cook the lasagna noodles according to package instructions until just al dente. This process usually takes about 8-10 minutes. Once cooked, drain the noodles and lay them flat on a clean kitchen towel to prevent sticking while you assemble your lasagna.

In a baking dish, spread a thin layer of the prepared tomato sauce at the bottom to prevent sticking. Lay down a layer of noodles, followed by a generous portion of the ricotta mixture, a layer of roasted vegetables, and a sprinkle of shredded mozzarella. Repeat the layers until all ingredients are used, finishing with a top layer of noodles, sauce, and remaining mozzarella.

Cover the baking dish with aluminum foil and bake in the preheated oven at 375°F (190°C) for 25 minutes. After this time, remove the foil and continue baking for an additional 15 minutes, or until the top is golden and bubbly, signaling that your delicious vegetarian lasagna is perfectly cooked.

Once out of the oven, allow the lasagna to rest for 10–15 minutes. This step is crucial as it helps the layers set and makes slicing easier. Cut into squares and serve warm, savoring the comforting flavors and textures of your deliciously easy vegetarian lasagna with roasted veggies.