Preheat oven to 425 degrees. Place onion, garlic, cauliflower, peppers, thyme, and rosemary on a sheet pan. Drizzle with 2 tablespoons olive oil, toss to coat, and season generously with salt and pepper. Transfer to oven and roast, shaking pan once or twice to encourage even coloring, until vegetables are deeply golden and crisp, about 40 minutes. Remove sheet pan from oven and place chicken breasts on top of vegetables. Season both sides with salt and pepper, drizzle with remaining 1 tablespoon olive oil, and return to oven. Roast until chicken is just cooked through, about 15 minutes. Serve, with lemon wedges, if desired.
Make Colorful Rice:
Combine rice, turmeric, cumin, 1 1/2 teaspoons salt, and 2 cups of water in a saucepan with a tight-fitting lid. Bring to a boil over high heat. Reduce to low, cover pot, and cook for an additional 35 minutes, until rice is tender. Remove from heat and let steam for an additional 5 minutes with the lid on. Uncover, fluff with a fork, toss with olive oil, and add cilantro. Season to taste with salt, and serve.