Roasting the chicken on a preheated pan saves time and helps the meat brown. You don't need to add oil to the vegetables; they cook in the drippings from the chicken.
Place a rimmed baking sheet in oven and preheat to 450 degrees, with racks in upper and middle thirds. Season chicken with salt and pepper. Carefully remove sheet from oven and arrange chicken, skin side up, in a single layer. Roast on upper rack, 20 minutes.
Remove chicken from oven and, with tongs, transfer to another rimmed baking sheet along with tomatoes. Add cauliflower, onion, and chickpeas to drippings on first sheet and toss to combine; season with salt and pepper. Roast chicken on upper rack and vegetables on middle rack until chicken is golden and cooked through and cauliflower is tender when pierced with a knife, about 30 minutes. Transfer chicken and vegetables to a serving dish and serve immediately.