Position a rack in the upper third of the oven and preheat to 400 degrees . On a rimmed baking sheet, toss the chicken with 1 tablespoon olive oil, the thyme and 1/2 teaspoon each salt and pepper; roast for 20 minutes. Meanwhile, in a medium bowl, combine the mushrooms, onion, peas and the remaining 1 tablespoon olive oil; season with salt and pepper. Add the vegetables to the chicken and cook for 25 minutes more.