Roast the vegetables: Preheat the oven and spread the zucchini, eggplant, bell peppers, and red onion in a single layer on a baking tray. Drizzle with olive oil, sprinkle evenly with salt and black pepper, and toss gently to coat. Roast until the vegetables are soft inside and lightly caramelized on the edges. This step removes excess moisture and concentrates their natural sweetness, which is key for a firm lasagna.

Prepare the cheese layer: In a mixing bowl, combine ricotta cheese, egg, grated Parmesan, and chopped basil. Stir until the mixture is smooth and well blended. The egg helps bind the layers, while Parmesan adds depth and savory balance to the creamy filling.

Build the base: Lightly grease the baking dish, then spread a thin, even layer of marinara sauce across the bottom. This prevents the lasagna sheets from sticking and ensures the bottom layer cooks evenly without drying out.

Start layering: Place a single layer of lasagna sheets over the sauce, slightly overlapping if needed. Spoon over some roasted vegetables, spread a portion of the ricotta mixture evenly, add a layer of marinara sauce, and finish with a sprinkle of mozzarella. Each layer should be balanced, not overloaded.

Repeat carefully: Continue layering in the same order until all ingredients are used, finishing with sauce and a generous topping of mozzarella on the final layer. Keeping the layers even helps the lasagna bake uniformly and hold its shape.

Bake until set: Cover the dish loosely with foil and bake until the lasagna is heated through and the layers have melded together. Remove the foil during the final minutes to allow the cheese on top to melt fully and develop a light golden color.

Rest before slicing: Remove the lasagna from the oven and let it rest for at least 15 minutes. This settling time allows the layers to firm up, making the lasagna easier to slice and serve clean portions.