How to Make It
Adjust oven rack to lowest position. Heat oven to 450° F. Place the mushrooms, garlic, and thyme in a large ovenproof skillet or small roasting pan, drizzle with 1 tablespoon of the oil, and toss to coat. Place the wire rack on top of the vegetables.
Pat the chicken dry and place it in the pan. Tuck the wings beneath the chicken, as if it were placing its arms behind its head. Rub the remaining tablespoon of oil over the skin. Sprinkle with 1/2 teaspoon of the salt and the pepper. Toss the remaining salt into the cavity.
Roast for 20 minutes. Add 1 cup water and roast until the chicken is cooked through and the thigh meat registers 180° F on a meat thermometer, about 40 minutes more. Use a spatula to loosen the chicken. With a wad of paper towels in each hand, transfer the bird to a cutting board. Let it rest for at least 15 minutes before carving.
Place the pan containing the drippings over medium heat. Add the wine and tomatoes (if using) and cook, stirring and scraping the bottom of the pan, until the sauce reduces slightly, about 5 minutes. Meanwhile, combine the cornstarch with 11/2 teaspoons water in a small bowl. Add the cornstarch mixture to the pan with the mustard and cook until the sauce thickens. Remove from heat. Spoon the sauce over the carved chicken