Heat oven to 425°F. Lay eggplant slices on two rimmed baking sheets in a single layer. Brush both sides with 2 tablespoons olive oil and sprinkle with salt. Roast until tender and lightly golden, 20-25 minutes, flipping halfway through. This step draws out moisture so your lasagna won't be watery.

While eggplant roasts, heat remaining 1 tablespoon olive oil in a large skillet over medium-high heat. Add ground beef and cook, breaking it up, until browned and cooked through, about 6-8 minutes. Add diced onion and cook until softened, 4 minutes. Stir in garlic and cook 30 seconds until fragrant.

Add crushed tomatoes, oregano, red pepper flakes if using, and 1/2 teaspoon salt. Bring to a simmer, then reduce heat to medium-low and cook until slightly thickened, about 10 minutes. Taste and adjust seasoning.

In a medium bowl, combine ricotta, egg, chopped basil, 1 cup mozzarella, and 1/4 cup Parmesan. Stir until well blended.

Reduce oven to 375°F. Spread 1 cup meat sauce in the bottom of a 9x13 inch baking dish. Arrange half the roasted eggplant slices to cover, overlapping slightly. Spread half the ricotta mixture over eggplant, then 1 1/2 cups meat sauce. Repeat layers: remaining eggplant, ricotta, and sauce. Top with remaining 1 cup mozzarella and 1/4 cup Parmesan.

Cover loosely with foil and bake 25 minutes. Remove foil and bake until cheese is melted and lightly browned in spots, 10-15 minutes more. Let rest 10 minutes before slicing so the layers set. Garnish with fresh basil and serve.