In a large saucepan, saute onion and mushrooms in butter until onion is tender; remove with a slotted spoon and set aside. In a small bowl, whisk flour and broth until smooth; gradually add to the pan. Bring to a boil; cook and stir until thickened, 1-2 minutes. , Add the roast beef, garlic powder, paprika, pepper, salt, pepper sauce and onion mixture; cook and stir until heated through. Garnish with cheese if desired.