Saute mushrooms and onions in the butter in a large saucepan. Sprinkle with the flour and cook and stir until the flour is lightly browned. Add the milk and half of the sour cream and continue to cook and stir over low heat until the mushrooms are tender. Add the seasonings and remaining sour cream.
Serve hot with crackers. Transfer to a fondue pot over low heat or a heat-proof dish on a warming tray.