In a large pan heat butter and oil over medium heat. Add the onions and cook until they begin to get soft. Add the mushrooms and garlic. Cook until soft. Add the rice to the pan and toast for 2 minutes stirring constantly. Add the thyme, sherry and worcestershire. Turn the heat to medium low. Begin adding the chicken stock. Add enough to cover the rice. Bring to a simmer, stirring frequently. When the stock has absorbed add more stock. Continue this process until the rice is tender, creamy, and almost all of the liquid has been absorbed. Add 1 tbsp butter, season with salt and pepper. Cool completely.