Slice your eggplants 1/3 – 1/2 inch (1 cm) thick, mix them with 3 teaspoons of salt. Put them into a large bowl and allow them to rest for at least 30 minutes. Wash them with running water and pat them dry with paper towel.
Cook the eggplant slices for 3 minutes on both sides in 1-1 tablespoon of olive oil per batch over high heat.
Preparing the mushrooms
Wash and clean the mushrooms. Slice them into 1/6 – 1/5 inch (4-5 mm) slices. Finely chop 2 whole onions.
Fry the onions with the olive oil over medium heat for 3-4 minutes until translucent. Add the mushrooms and stir fry them over high heat until they evaporate all their liquids. This may take 3-6 minutes. Add salt and pepper. You may need to fry your onions and mushrooms in 2-3 batches.
Preparing the tomato sauce
Finely chop the onion and garlic. Cook the onion over medium-low heat with 1 tablespoon of olive oil for 3-4 minutes until translucent. Add the garlic and cook for 30 seconds.
Add the tomato purée and some water (2-3 tbsp). Season with salt, pepper, and oregano or basil. Bring it to a boil and simmer over low heat for 30 minutes. Stir occasionally until the sauce thickens.
Precook the lasagne pasta
Bring a large amount of water to a boil in a larger pot. Add salt to the water – use a fair amount of salt.
Put the lasagna sheets into the water to precook them. Stir them occasionally, particularly at the beginning, or else they stick together. I cooked my 24 sheets in two portions.
Cook for about 5-6 minutes then transfer them to a plate. They won’t be completely soft, but they will continue cooking in the oven. Reserve the cooking water for later use.
Assembling and baking the vegan lasagna
Preheat the oven to 350 °F (180 °C).
Spread 4 tbsp of tomato sauce evenly on the bottom of a 9 x 13 inch (25×35 cm) pan.
Put one layer of lasagna on the bottom of your pan. Sprinkle the top of the pasta layer with 2-3 tbsp of the cooking water.
Spread half of the mushrooms on top of your first lasagna layer.
Put another layer of pasta then spread 8 tbsp of tomato sauce onto the top. Make sure to spread the sauce so that it covers the entire layer, it keeps the pasta from drying out.
Place half of the eggplant slices on top of your lasagna layer.
Repeat layering in the same order; lasagna pasta » 2-3 tbsp of cooking water » remaining half of mushrooms » 8 tbsp of tomato sauce » remaining part of eggplant slices » lasagna » 8 tbsp tomato sauce.
Cover with aluminum foil and bake for 20 minutes. Remove foil and continue baking for an extra 20 minutes. Bring 2 cups (500 ml) of water to a boil in an oven-safe skillet and put it onto the bottom of your oven; The evaporating water will keep your lasagna from drying out.