In a large saucepan, heat butter over medium-high heat; saute mushrooms and onion until tender., Mix flour, salt, pepper and 1 can broth until smooth; stir into mushroom mixture. Stir in remaining can of broth. Bring to a boil; cook and stir until thickened, about 2 minutes. Reduce heat; stir in cream. Simmer, uncovered, until flavors are blended, about 15 minutes, stirring occasionally.