Combine 1 1/2 pounds diced tomatoes, 1 sliced onion, 1 sliced garlic clove, 3 tablespoons olive oil, 2 teaspoons salt, and 1 teaspoon sugar in a large bowl and toss to combine. Spread mixture evenly over eggplant. Cover with foil and roast for 40 minutes or until eggplant is knife tender. Remove foil and continue roasting until onions are browned and juices are reduce by half, about another 30-40 minutes.