Heat oven to 400° F. Rinse the chicken and pat it dry with paper towels. Place in a roasting pan. Drizzle with 1 tablespoon of the oil and season with 1 1/2 teaspoons of the salt and 1/2 teaspoon of the pepper. Rub to coat. In a large bowl, combine the potatoes, rosemary, tomatoes, mushrooms, and the remaining oil, salt, and pepper. Scatter the potatoes around the chicken. (If the potatoes don't fit in a single layer, place some under the chicken.) Roast until the chicken is cooked through and a thigh registers 180° F on a meat thermometer, about 45 minutes.