Preheat the oven to 400 degrees F. Cut the eggplant, bell pepper, and onion into cubes. Toss them in a large bowl with the garlic, olive oil, salt, pepper and pepper flakes. 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. Add chopped basil and lemon zest & juice. Taste for salt and pepper.