Start by preparing the tomato sauce. In a pot add the olive oil, eggplant and 2 cloves of minced garlic. Cook it in medium-high for around 3 minutes.
Add the 2 cans of tomato sauce, salt, black pepper, sugar, basil and water. Mix everything well and then reduce the heat to low. Let the sauce simmer for 20 minutes.
While the tomato sauce is simmering, we can start preparing the vegetables. In a medium heat pan, add ½ tablespoon of olive oil and spread it well around the pan. Add the zucchini slices. Cook each side for around 4 minutes. Add a pinch of salt and pepper on each side. Remove them from the pan and reserve.
In the same pan, add ½ tablespoon of olive oil, the 1 clove of finely chopped garlic and the mushrooms. Add a pinch of salt and pepper. Cook them for around 2-3 minutes. Set them aside.
Time to make the white sauce. In a medium-high heat pot, add the butter and the flour. Mix it well. Add the milk, a pinch of salt and pepper and keep mixing it until you reach a creamy consistency. It will take around 5 minutes.
Now it is time to check the tomato sauce that is simmering. Adjust the seasoning as you like it (more salt, pepper) and if it is still very acidic, add ½ teaspoon of baking soda. It will remove the acidity immediately.
All the layers are ready. Time to build the lasagna. There is no need to precook the dried lasagna sheets. One tip is to let the tomato sauce rest for some minutes (or cook it on the day before) so it is not super hot when you assemble the lasagna. When the tomato sauce is too hot, the dried lasagna sheets get curvy and make it harder to assemble it.
In your baking tray, add a bit of the tomato sauce at the bottom. Then place the first lasagna sheet layer. Another layer of sauce. Another layer of lasagna sheet. Tomato sauce again.
Now on top of the tomato sauce, make a layer with the grilled zucchinis.
Add a layer of mozzarella cheese.
Add tomato sauce on top of the cheese. Another layer of lasagna sheets, and the tomato sauce. Make a layer with the grilled mushrooms. A little bit of tomato sauce. The last layer of lasagna sheets. On top of everything add the white sauce and the parmesan for last.
Put the lasagna into a preheated oven at 180°Celsius (350°Fahrenheit) with a foil paper for 30 minutes.
Then remove the foil, and bake it until the parmesan cheese is melted and a bit crunchy (it takes around 15 minutes).
Let the lasagna cool for around 15 minutes before serving. Enjoy! :)