1. Warm one-quarter of the oil in a large, heavy-bottomed pot over medium-high heat. Saute the onions and peppers until brown and soft (8-10 minutes). Transfer to a large bowl and set aside.
2. Add one-quarter of oil to the pot and saute the zucchini until soft (2-3 minutes). Add to onions and peppers.
3. Add one-quarter of oil to the pot and saute eggplant until translucent (5-6 minutes). Add to cooked vegetables.
4. Add remaining one-quarter of oil to the pot and saute the garlic until fragrant (1 minute). Stir in the tomatoes, bay leaf, thyme, and salt.
5. Return all vegetables to the pan. Simmer, stirring occasionally, until all vegetables are tender and mixture is slightly creamy (1 hour). Remove bay leaf and thyme. Add pepper to taste and stir in the basil to serve.