Preheat your oven to 400ºF.
Slice the top and bottom off the eggplant, then stand it up on a cutting board and slice it into 12 thin slices, about 1/8-inch thick. No need to peel.
Arrange the eggplant on two large baking sheets lined with parchment paper, sprinkle them with salt, and bake until tender about 20 minutes.
While the eggplant is roasting, prepare the mushrooms. Finely dice them and heat the olive oil in a large skillet over medium-high heat. Sauté the diced mushrooms with the garlic, oregano, a 1/2 teaspoon of salt, and a 1/4 teaspoon ground black pepper. The mushrooms should release moisture after 8 to 10 minutes.
Pour the ricotta into a mixing bowl, and add the egg, and a 1/4 teaspoon of salt and pepper. Mix well, and set aside.
Remove the eggplant from the oven and let it cool briefly. Add 3/4 cup of marinara sauce to the bottom of a 9-by-13 inch baking dish.
Add one layer of eggplant slices, using about 4 large pieces. You don't have to be perfectly about filling any gaps in the dish.
Spread half of the ricotta mixture over the eggplant, followed by half of the mushroom mixture. Top with 1 cup of shredded mozzarella cheese.
Repeat the layers, starting with a new layer of eggplant and another 3/4 cup of marinara sauce. Add the remaining ricotta mixture, followed by the remaining mushrooms. Add a final layer of eggplant on top, then top with the remaining marinara sauce and shredded mozzarella cheese.
Bake at 400ºF until the top is golden and bubbly, about 40 to 45 minutes.
Let the lasagna cool for 15 minutes, then slice and serve warm.
