Step 1

Preheat the oven to 375°F (190°C).

Step 2

Heat the olive oil in a 10-inch cast-iron skillet over medium heat. Add the red onion, garlic, and ¾ teaspoon of salt.

Step 3

Cook, stirring often until the onion has softened, about 10 minutes.

Step 4

Add the bell pepper and cook, stirring occasionally, until softened, about 7 minutes. Pour in the crushed tomatoes and season with the herbes de Provence and 2 teaspoons of thyme.

Step 5

Cook at a low simmer until the sauce has slightly thickened, 10 to 15 minutes. Add the basil and stir to combine. Remove from the heat.

Step 6

Starting from the outer edge of the skillet and working your way to the middle, arrange the sliced veggies over the tomato sauce, snugly upright but slightly angled, and alternate the colors (zucchini, summer squash, eggplant, and tomato). Fan them apart slightly, if needed, to cover the entire pan with no large gaps.Sprinkle with the remaining 1 teaspoon thyme, remaining ¼ teaspoon salt, and the black pepper.

Step 7

Cover the skillet with foil or an oven-safe lid and bake the ratatouille for 40 minutes.

Step 8

Remove the foil and continue cooking until the vegetables are very soft and the ratatouille is bubbling, another 20 to 30 minutes.

Step 9

Garnish with basil leaves just before serving, if desired.