Heat the olive oil in a large saute pan over medium-high heat until it begins to shimmer. Add the onion, carrot, celery, garlic, thyme leaves, and bay leaf and saute until the onion turns translucent. Add the zucchini and summer squash, and salt and black pepper, to taste. Stir to coat with the onion mixture. Sprinkle red bell pepper over squash. Reduce heat to medium-low. Cover and cook until tender about 20 minutes. Transfer to a serving dish and garnish with fresh basil.