Cut the eggplant, bell pepper, and onion into 1-inch cubes. Toss them in a large bowl with the garlic, olive oil, salt, and pepper. Spread them on a baking sheet. Roast for 45 minutes, until the vegetables are lightly browned and soft, tossing once during cooking.
Cool slightly.

Place the vegetables in a food processor fitted with a steel blade, add the tomato paste, and pulse 3 or 4 times to blend. Taste for salt and pepper.