Heat your olive oil in a large skillet over medium heat. Add the diced onion and cook until it turns soft and translucent, around 3 to 4 minutes. You'll hear it sizzle slightly but don't let it brown.
Stir in the minced garlic and cook for exactly 1 minute — no longer or it gets bitter. The kitchen smells amazing at this point, I know.
Add your diced eggplant to the pan and cook for 5 minutes, stirring occasionally. The eggplant will start to soften and absorb all that garlicky oil. Don't panic if it looks a little dry at first.
Now add the bell peppers and zucchini, then stir everything together. Pour in the diced tomatoes along with the thyme, basil, bay leaf, salt, and pepper. Mix well so all the vegetables are coated.
Reduce the heat to medium-low and simmer for 20 to 25 minutes. Stir occasionally and watch it thicken — the tomatoes will break down and create that gorgeous sauce. Don't crank the heat up or the vegetables get mushy.
Taste your summer ratatouille family French mixture and adjust seasoning if needed. Remove the bay leaf before serving. Mine always sticks a little to the pan bottom and that's actually fine — those caramelized bits add flavor.