Heat oven to 375°F. Cut 2 (25x12-inch) pieces of foil. Slightly crush each sheet to make 3 1/2-inch ball; flatten balls slightly. Place on ungreased cookie sheet.
Place biscuits on microwavable plate. Microwave uncovered on High 30 to 40 seconds, turning biscuits over halfway through microwave time, until soft enough to press into rounds. Press or roll each biscuit to 6-inch round. Place 1 biscuit round over each foil ball, shaping biscuit gently to fit. Bake 12 to 16 minutes or until golden brown. Cool 5 minutes. Remove biscuit cups from foil balls.
Meanwhile, in 2-quart saucepan, melt butter over medium-high heat. Add onion and mushrooms. Cook and stir 5 to 7 minutes or until vegetables are tender. Stir in soup. Reduce heat to medium; cook about 4 minutes, stirring occasionally, until hot.
In small bowl, stir cold water, cornstarch and pepper until smooth. Increase heat to medium-high; stir cornstarch mixture into soup mixture in pan, cooking and stirring until mixture boils. Cook, stirring constantly, about 1 minute or until mixture is slightly thickened.
Place biscuit bowls in individual shallow soup bowls; spoon half of the beef stew into each. (Some soup may flow over side of biscuit bowl.)