Step 1: Prepare the Sauce
Preheat oven to 350°F (175°C).
Quarter the bell peppers and remove seeds and stems. Peel and quarter the onions. Quarter the tomatoes and peel the garlic cloves.
Arrange onions, peppers, garlic, and tomatoes on a parchment-lined baking tray. Add rosemary, olive oil, salt, and black pepper. Toss well.
Roast for 20 minutes until vegetables are soft and fragrant.
Remove rosemary sprigs and blend the roasted vegetables until smooth.
Step 2: Prepare the Vegetables
Bring a pot of water to a boil and prepare a bowl of ice water.
Score small crosses on the tomatoes and boil for 1 minute. Transfer immediately to ice water.
Peel the tomatoes and slice them thinly.
Thinly slice zucchini and eggplants using a mandoline or sharp knife.
Step 3: Assemble the Ratatouille
Spread part of the sauce into the bottom of a baking dish.
Arrange sliced vegetables upright in overlapping rows, alternating colors.
Brush with olive oil and season with salt and black pepper.
Step 4: Bake the Ratatouille
Cut parchment paper to fit over the baking dish and make a small hole in the center.
Cover the vegetables with parchment paper and bake for 30 minutes.
Remove parchment paper and bake for another 20 minutes until tender and lightly caramelized.
Serve with extra sauce, chopped parsley, and fresh chives.