Preheat your oven to 400°F (200°C) or 180°C for a fan oven. Line 2 large rimmed baking sheets with parchment paper. Prepare a 9x13-inch (approx 23x33cm) casserole dish.
Arrange the eggplant slices on the baking sheets in a single layer. Brush both sides with 2 tablespoons of olive oil and season with 1 tablespoon of kosher salt.
Bake the eggplant in the oven for 15 minutes, flipping halfway through, until tender. Remove from oven and reduce the heat to 375°F (190°C) or 170°C for a fan oven.
In a large bowl, combine ricotta cheese, chopped spinach, egg, ½ cup of mozzarella cheese, ¼ cup of parmesan, dried oregano, and black pepper. Set aside.
Heat 1 tablespoon of olive oil in a skillet. Sauté the mushrooms until tender, then add garlic and cook for 1 more minute. Stir in the marinara sauce and remove from heat.
Spread a small amount of the marinara mixture at the bottom of the casserole dish. Layer with half of the eggplant slices, half of the ricotta mixture, and half of the marinara mixture.
Repeat the layers with the remaining eggplant, ricotta mixture, and marinara mixture.
Sprinkle the remaining mozzarella and parmesan cheese on top. Bake in the oven for 30 minutes, or until the lasagna is hot, bubbly, and the cheese is melted.
Allow the lasagna to rest for 10 minutes. Slice and serve.