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Cream of mushroom soup is a simple type of soup where a basic roux is thinned with cream or milk and then mushrooms or mushroom broth are added. In North America, it is a common canned condensed soup. Cream of mushroom soup is often used as a base ingredient in casseroles and comfort foods. This use is similar to that of a mushroom-flavored gravy. Soups made with cream and mushrooms have been made for many hundreds of years, based on French cream sauces. In America, the Campbell Soup Company began producing its canned Cream of Mushroom Soup in 1934. Home cooks had already been using canned soup as a casserole or sauce base, and Campbell's started publishing its own recipes based on it in 1941.

Source: Wikipedia