Heat oven: To 425°F (220°C) degrees.
Roast eggplant: Transfer eggplant slices to paper towels and blot them of as much excess water as possible. Dry the baking sheet, too, and coat it with 3 tablespoons of olive oil and sprinkle the oil with several grinds of black pepper. Arrange eggplant slices in one layer. Drizzle with 2 more tablespoons of oil and more grinds of black pepper. Roast until nicely browned underneath, 20 to 25 minutes. Use a thin spatula to turn eggplant slices over and roast until browned underneath on the second side, another 10 minutes. It’s okay if they’re not cooked through yet. Reduce oven temperature to 375°F (175°C).
Meanwhile, make sauce: While eggplant roasts, open your can of tomatoes and use kitchen shears or very well-washed scissors to chop the tomatoes into small bits. Heat 2 tablespoons olive oil in a medium/large saucepan over medium-high heat. Once hot, add onion and cook until it softens, about 8 minutes. Add garlic and pepper flakes and cook another minute. Add tomatoes (beware the splatter!), oregano, and season with 1 teaspoon salt, plus more to taste. Let mixture simmer over low heat for about 10 minutes, stirring occasionally. If you’d like the sauce smoother than it is now, you can mash the tomatoes further with a potato masher as it cooks, or use an immersion blender for a completely smooth sauce. Stir in basil then taste the sauce; adjust salt and pepper to taste.
Assemble: In a 1.5- to 2-quart baking dish (holding 6 to 8 cups; I’m using this), spoon about 1/2 cup of the sauce. Arrange about half of the eggplant slices, slightly overlapped, in one layer. Spoon another 1/2 cup sauce over and spread it evenly. Arrange remaining eggplant slices, then ladle more sauce over it so it’s as saucy as you like. [Keep unused sauce in fridge for another day; it will keep for 4 to 5 days.] Sprinkle top with mozzarella and parmesan.
Bake: For 30 minutes, until cheese has melted and sauce is bubbling all over. For more color on top, you can run it briefly under your oven’s broiler.
To serve: Once baked, let the eggplant parmesan rest at room temperature, if you can bear it, for 15 minutes before digging in — this helps the ingredients settle and come together more. It’s not mandatory, but this is excellent with a side of garlic bread.
Do ahead: Eggplant parmesan keeps fantastically in the fridge for up to 5 days. Rewarm in a 350-degree oven, covered for the first half of the time, for about 25 to 30 minutes. You can also freeze it, well wrapped, for a few months.