In a large skillet, bring the marinara sauce, mushrooms, olives, tomatoes, Italian seasoning and pepper flakes to a low boil. Simmer while cooking sausage. Heat 1 tablespoon olive oil over medium heat. Add sausage and cook until no longer pink; drain. Chop briefly in food processor to break into small pieces. Stir into sauce; continue to simmer.
In a shallow bowl, whisk together eggs and buttermilk. Place breadcrumbs in another bowl. Slice eggplant around 1/4-inch thick and dip each slice into egg mixture, then into breadcrumbs. Heat 2 tablespoons olive oil in over medium heat. Sauté eggplant until golden brown on each side; add additional oil as needed. Place on paper towel to drain.
Preheat oven to 400 degrees. Spread some marinara sauce on bottom of a 9-by-13 inch baking dish (to cover the bottom). Layer eggplant on top in 2 to 3 rows. Cover eggplant with remaining sauce. Mix together two cheeses. Sprinkle over top of sauce. Bake for 30 - 40 minutes or until bubbly, cheese is melted and golden brown. Let sit 5 minutes before serving.