Preheat oven to 350 °F (180 °C).
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. 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 don’t worry, they will continue softening in the oven. That’s how you get a perfect pasta consistency! Reserve some of the cooking water – see later. 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 » 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.