Before you roast the chicken, place the mushrooms, garlic, and thyme in the pan, drizzle with the oil, and toss to coat. Place the wire rack and the bird on top. Roast according to the recipe.
After you transfer the roast chicken to a cutting board, 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 1 1/2 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.