Chef Boulud's vibrant Provençal vegetable stew is a celebration of fresh produce. Sautéing the vegetables separately according to their respective cooking times ensures they are perfectly tender before baking together at the end. You can serve ratatouille warm or cold; it reheats beautifully as a vegetarian main or a hearty side for roast chicken, beef, or fish.
Preheat oven to 350 degrees F. Fill a large pot with water and bring to a simmer over high heat. Meanwhile, prepare vegetables: Peel and trim onions, then cut in half from top to bottom. Cut each half into 4 wedges, then cut into a large dice. Place in a bowl and set aside. Trim and core the peppers; discard stems and cores. Cut in half, then remove and discard seeds and veins. Slice into 1-inch strips, then cut into a large dice. Place in the bowl with the onions and set aside. Peel and slice garlic cloves in half. Remove and discard any green germs inside, then finely mince. Place in a bowl and set aside.
Trim zucchini and cut into quarters, lengthwise; then cut into 1-inch chunks. Place in a bowl and set aside. Trim eggplants, then cut in half. Cut each half into 3 or 4 wedges lengthwise, then cut into 1-inch chunks. Place in a bowl and set aside.
Sauté vegetables: In a Dutch oven over medium-high heat, add 2 tablespoons olive oil, followed by onions and peppers. Stirring occasionally, gently sweat the vegetables until they are soft but have taken on no color, 4–5 minutes. Meanwhile, in a skillet over medium-high heat, heat a generous tablespoon olive oil. Add zucchini, a pinch of salt, and a few turns of the pepper mill. Stirring occasionally, sauté until zucchini has taken on a golden color, 5–6 minutes. Season onion-pepper mixture with a few pinches of salt and the red pepper flakes.
Blanch tomatoes: When the large pot of water is simmering, prepare an ice bath by filling a large bowl with ice water; set aside. Use a paring knife to cut out the tomato stems and discard. Score an “x” into the bottom of each tomato, for easy peeling after blanching. Place tomatoes in simmering water for 30 seconds. Use a strainer to immediately transfer the tomatoes to the ice bath; allow to cool completely. Use a paring knife to peel away the tomato skin and discard.
When onion-pepper mixture has been sweating for 4–5 minutes, turn heat to low and cook for another 2–3 minutes. Remove zucchini from skillet, leaving some oil behind, and set aside. In the same skillet over medium-high heat, add another tablespoon olive oil, followed by eggplant, a pinch of salt, and a few turns of pepper. Stirring occasionally, sauté until eggplant has taken on a golden color, 5–6 minutes. Try to keep the eggplant pieces intact. (If the eggplant sticks, add up to another tablespoon or so of olive oil, as needed.)
Meanwhile, cut the tomatoes in half horizontally. Use your hands to remove most of the seeds, then gently squeeze to remove excess juice. Halve the tomatoes horizontally again, then cut into 1-inch chunks and set aside. Remove eggplant from heat and set aside; the volume of the eggplant should be reduced by about half. By now, the onions should be soft and translucent; stir garlic and tomato paste into onion-pepper mixture. Add zucchini, eggplant, and tomatoes; gently stir to combine. Season with 2 pinches of salt and bring to a low simmer, 2–3 minutes.
While the ratatouille is simmering, make a bouquet garni: Gather the basil, thyme, rosemary and bay leaf into a small bundle, using the basil to contain the other herbs. Wrap securely with butcher's twine and tie with a double knot. Trim the ends of the bouquet garni, then submerge it in the middle of the ratatouille. Next, make a parchment paper lid, or “cartouche,” for the Dutch oven: Fold a large sheet of parchment paper into quarters. Fold in half twice diagonally to make a flat wedge; then trim the outer edge so that the wedge is as long as the radius of the pot. Finally, snip off the point of the wedge to create a steam vent. Unfold the parchment: you should have a cartouche that is about the same size and shape as the Dutch oven, with a small hole in the center. Lay the cartouche directly on the surface of the ratatouille. Place in the oven and bake, 45–60 minutes.
After 45 minutes, remove ratatouille from oven; the vegetables should be soft and the sauce thickened. (If the ratatouille is watery, continue cooking for up to 15 more minutes.) Discard cartouche. Tear half the basil leaves and gently stir them into the ratatouille. Remove and discard the bouquet garni. Transfer the vegetables and most of the liquid to a serving platter. Garnish with whole basil leaves and baste the top of the ratatouille with remaining liquid. Taste and adjust seasoning as needed before serving, hot or cold.