For the vegetable filling
Bring a pot to medium heat, add the olive oil and sauté the onions, carrots and peppers.
Season with salt and cook until the vegetables soften.
Add the zucchini and sauté until it softens too.
Finally, drop in the garlic and cook until fragrant.
Add the tomato sauce, lower the heat, season with salt again and sugar if needed.
Simmer, stirring regularly, for about 20 minutes.
Add the basil and the tomato concassé, remove from heat and set aside to cool slightly.
Add the cheese and stir well.
For the lasagna
In a 24x30 cm baking dish, spread the tomato sauce and place over it a layer of lasagna.
Spread on top of the lasagna half the filling and repeat the process until you have 3 layers of lasagna and 2 layers of filling.
Coat with the remaining tomato sauce and sprinkle with parmesan.
Bake in a preheated oven at 170 degrees C, until the parmesan turns gold, for about 15 minutes.