In a large skillet cook the onion and the garlic in 2 tablespoons of the oil over moderately low heat, stirring occasionally, until the onion is softened. Add the remaining 3 tablespoons oil and heat it over moderately high heat until it is hot but not smoking. Add the eggplant and cook the mixture, stirring occasionally, for 8 minutes, or until the eggplant is softened. Stir in the zucchini and the bell pepper and cook the mixture over the moderate heat, stirring occasionally, for 12 minutes. Stir in the tomatoes and cook the mixture, stirring occasionally, for 5 to 7 minutes, or until the vegetables are tender. Stir in the oregano, the thyme, the coriander, the fennel seeds, the salt, and pepper to taste and cook the mixture, stirring, for 1 minute. Stir in the basil and combine the mixture well. The ratatouille may be made 1 day in advance, kept covered and chilled, and reheated before serving.