Preheat the oven to 425°F (220°C). Line a large rimmed baking sheet with parchment paper.

Slice the 2 1/2 pounds eggplant lengthwise into planks about 1/4 inch thick. Arrange them in a single layer on the prepared baking sheet. Brush both sides with the 3 tablespoons olive oil, then season with the 1/2 teaspoon kosher salt and 1/4 teaspoon black pepper.

Roast for 20–25 minutes, turning the slices halfway through. They should be tender and lightly browned at the edges. If the slices still look firm in the center, roast for another 3–5 minutes. Let them cool slightly while you prepare the other layers.

Reduce the oven temperature to 375°F (190°C). Heat the 2 tablespoons olive oil in a large skillet or saucepan over medium heat. Add the 4 minced garlic cloves and cook for about 30 seconds, until fragrant.

Stir in the 24 ounces marinara sauce, 1 teaspoon Italian seasoning, and optional 1/4 teaspoon crushed red pepper flakes. Simmer over medium-low heat for 8–10 minutes, stirring occasionally. The sauce should be thick enough to coat a spoon. If it looks watery, continue simmering for a few more minutes.

In a medium bowl, combine the 15 ounces ricotta, 1 beaten egg, 1/2 cup Parmesan, 2 tablespoons parsley, 1/4 teaspoon kosher salt, and 1/4 teaspoon black pepper. Stir until evenly combined and creamy.

Spread about 1/2 cup of the tomato sauce over the bottom of a 9x13-inch baking dish. Arrange a layer of roasted eggplant over the sauce, slightly overlapping the pieces if necessary.

Spread about one-third of the ricotta mixture over the eggplant. Sprinkle with some of the 2 cups shredded mozzarella. Add another layer of eggplant, followed by sauce, ricotta, and mozzarella. Repeat until the ingredients are used, finishing with a layer of tomato sauce.

Sprinkle the remaining mozzarella over the top, followed by the 1/4 cup Parmesan. The surface should be evenly covered but not packed down.

Cover the baking dish loosely with aluminum foil, making sure the foil does not press directly into the cheese. Bake at 375°F (190°C) for 30 minutes.

Remove the foil and continue baking for 15–20 minutes, until the sauce is bubbling around the edges and the cheese is melted and golden brown. For deeper browning, you can broil for 1–2 minutes, watching constantly.

Remove the lasagna from the oven and let it rest for 15 minutes before slicing. This gives the layers time to firm up. Cut into 8 portions and serve warm.