You can use a mixture of eggplant, zucchini, mushrooms and spincah or any combination of the four. You can use only one, or two of them or all of them. Totally up to you.
Peeling of the eggplant and zucchini is not necessary.
Slice eggplant and zucchini into 1/4 to 1/3 inch slices from top to bottom. I think 1/4" is too thin but 1/2 is too thick, so somewhere in between.
Lay slices of eggplant and zucchini in a flat dish and sprinkle with salt. Add another layer and some more salt until all the veggies have been salted. Let sit until the sauce is ready.
Remove veggies from salting dish and rinse veggies very well to get off all the salt. Place on toweling and pat tops to dry veggies. Remember to rinse the salt off.
Put a layer of sauce on the bottom of the dish and then add a layer of veggies. You need to add some cheese in with the veggies or they will fall apart and your lasagna will fall apart when you try to serve it.
Continue to layer the sauce, cheese and veggies just as you would with regular lasagna except use the veggies instead of the pasta.
Continue layering the lasagna until the dish is full, ending with the sauce.
Cover with foil and bake until it ia all hot and bubbly
Uncover and put cheese over top and return to oven, uncovered, until cheese has melted and is all gooey.
Remove from oven and allow to sit for 15 to 20 minutes.