Mix cornstarch and milk until smooth. Pound chicken with a meat mallet to 1/4-in. thickness., In a large nonstick skillet, heat oil over medium heat; cook chicken until no longer pink, 5-6 minutes per side. Remove from pan., In same pan, heat butter over medium-high heat; saute mushrooms and onion until tender. Stir in sherry, salt and pepper; bring to a boil. Stir cornstarch mixture and add to pan. Return to a boil; cook and stir until thickened, 1-2 minutes. Return chicken to pan; heat through.