Heat the olive oil in a large skillet or Dutch oven over medium heat until shimmering.
Add the sliced zucchini and bell peppers, sautéing them until they start to soften and become fragrant, about 5 minutes. They should develop a slight golden color and release a lovely aroma.
Add the cubed eggplant to the pan, stirring well to combine. Cook for another 7-8 minutes until the eggplant begins to soften and absorb the flavors, turning slightly golden.
Stir in the minced garlic and cook for about 1 minute until fragrant, filling the kitchen with a warm aroma. Be careful not to burn the garlic.
Add the chopped tomatoes along with thyme and bay leaf, stirring everything together. Increase the heat slightly to bring the mixture to a simmer, then reduce the heat to low.
Cover the skillet partially and let the vegetables simmer gently for 30-40 minutes, stirring occasionally. The vegetables will soften further, and the sauce will thicken, developing rich, integrated flavors.
Remove the bay leaf and taste the stew, seasoning with salt and black pepper as needed. Adjust the seasoning to your preference.
Serve the ratatouille hot, garnished with fresh herbs if desired. It pairs beautifully with crusty bread or rice for a complete meal.