Trim the ends of each eggplant and, if desired, peel the skin. Slice lengthwise into about 1/4-inch thick slices.
Arrange slices in a single layer on a cooling rack set over a baking sheet, sprinkle both sides with 2 teaspoons kosher salt, and let sit for 30 minutes to draw out moisture.
Pat the eggplant slices dry with paper towels. Alternatively, place salted slices on a baking sheet and bake at 375°F until slightly softened, about 20 minutes.
Preheat oven to 375°F (if you used the baking alternative earlier, keep the oven temperature).
Heat 1 tablespoon olive oil and 1 tablespoon butter in a large skillet over medium heat. Add 3 teaspoons minced garlic, the chopped onion, and chopped green pepper; sauté 3–4 minutes until softened.
Add 4 oz sliced baby Bella mushrooms and cook until the mushrooms soften, about 3–4 minutes.
Stir in 20 oz crushed tomatoes, 1.5 teaspoons Italian seasoning, 1/2 teaspoon dried oregano, 1/2 teaspoon black pepper, and 1/2 teaspoon salt; heat through and then set the sauce aside.
In a medium bowl, mix 16 oz ricotta cheese with 1 large egg and 1/2 cup Parmesan until combined.
Spread a thin layer of the tomato-mushroom sauce on the bottom of a 13 x 9-inch baking dish.
Place about 6 eggplant slices in a single layer over the sauce, spread half of the ricotta mixture over the eggplant, and sprinkle with one-third of the 2 cups shredded mozzarella.
Repeat the layer: another layer of eggplant, the remaining ricotta mixture, and another one-third of the mozzarella.
Top with the remaining eggplant slices, spread remaining sauce over them, and sprinkle with the last third of the mozzarella.
Cover the dish with aluminum foil and bake at 375°F for 25 minutes. Remove the foil and bake a few more minutes until the cheese is melted and bubbly.
Let the lasagna cool for 10 minutes, garnish with parsley, slice, and serve.
