Slice eggplants into 1/4-inch thick rounds. Optional: salt slices generously, let sit in a colander for 30 to 60 minutes, rinse thoroughly, and pat completely dry with paper towels.

Cook eggplant using your preferred method. Pan-fry: Heat 2 tablespoons olive oil in a large skillet over medium-high heat. Fry slices in batches for 3 to 4 minutes per side until golden and tender. Drain on paper towels. OR Bake: Preheat oven to 400°F. Arrange slices on parchment-lined baking sheets, drizzle with oil, and bake for 20 to 25 minutes, flipping halfway, until tender and lightly golden.

Make sauce: Heat 2 tablespoons olive oil in a large saucepan over medium heat. Cook chopped onion for 5 to 7 minutes until soft. Add minced garlic and cook for 1 minute. Stir in crushed tomatoes, tomato sauce, oregano, basil, and red pepper flakes. Season with salt and pepper. Simmer for 15 to 30 minutes, stirring occasionally.

Make ricotta mixture: Combine ricotta cheese, Parmesan, beaten egg, and chopped parsley in a medium bowl. Season with salt and pepper and mix until well combined.

Assemble casserole: Preheat oven to 375°F. Grease a 9x13-inch baking dish. Spread 1 cup of sauce on the bottom. Layer eggplant slices, half the ricotta mixture, half the mozzarella, and a third of remaining sauce. Repeat layers. Finish with final layer of eggplant, remaining sauce, and extra mozzarella and Parmesan.

Cover tightly with aluminum foil and bake for 30 minutes. Remove foil and bake for another 15 to 20 minutes until cheese is bubbly, golden brown, and sauce is bubbling around edges.

Let casserole rest for 10 to 15 minutes before slicing to allow layers to set.