Wash all the vegetables thoroughly before chopping them into evenly sized pieces. Keeping the vegetables similar in size helps them cook evenly without turning mushy. Fresh summer vegetables create the richest flavor and texture in this dish.
Heat olive oil in a large Dutch oven over medium heat. Add the sliced onion and cook until soft and lightly golden, then stir in the garlic. The aroma should become fragrant and slightly sweet after a few minutes.
Add chopped tomatoes and bell peppers into the pot and stir gently. Let them simmer until the tomatoes begin breaking down into a rustic sauce. This creates the flavorful base that ties the whole dish together.
Stir in the eggplant, zucchini, and yellow squash with thyme and bay leaf. Cook slowly so the vegetables soften while still holding their shape. Slow cooking allows the vegetables to absorb the herbs beautifully.
Reduce the heat and let everything simmer uncovered for about 25 minutes. Stir occasionally so the vegetables cook evenly and the sauce thickens naturally. The ratatouille should look rich, glossy, and hearty.
Remove the bay leaf and stir in freshly chopped basil right before serving. Fresh herbs added at the end keep the flavor bright and vibrant. Taste and adjust salt and pepper if needed.
Slice the crusty bread and lightly toast it in the oven until crisp and golden. Serve warm beside the ratatouille or place slices directly in the bowl. The bread absorbs the flavorful sauce perfectly.