Arrange the slices in a colander, sprinkle about 1/2 tsp salt over them, and let sit for 20–30 minutes. This draws out excess moisture and can reduce bitterness. After resting, rinse briefly and pat dry with paper towels.

Cook sauce and eggplant

Heat a large skillet over medium heat and add 2 tablespoons of olive oil. When shimmering, add the chopped onion and sauté until soft and translucent, about 5–6 minutes.

Add the minced garlic and cook 30–60 seconds until fragrant, stirring so it doesn't burn.

If using slices: push the onion/garlic to the side, add remaining 1 tablespoon olive oil, and add eggplant slices in a single layer (you may need to work in batches). Cook until golden and softened, about 4–5 minutes per side for slices. If using cubes: add the eggplant cubes now and sauté, stirring occasionally, until browned and tender, about 8–10 minutes.

Add the diced tomatoes (or 1 cup canned crushed tomatoes), salt, black pepper, and red pepper flakes if using. Stir to combine, lower the heat to medium-low, cover, and simmer for 8–12 minutes until the flavors meld and the eggplant is fully tender. Stir occasionally; if the pan gets too dry, add 2–3 tablespoons water.

Finish and serve

Taste and adjust seasoning with more salt, pepper, or a squeeze of lemon juice (about 1 tsp) for brightness.

Stir in chopped fresh basil or parsley just before serving. Sprinkle with grated Parmesan or crumbled feta if desired.

Serve warm as a side, over rice or pasta, or spooned onto toasted bread as a bruschetta-style topping.