In a small bowl, whisk soy sauce, sherry, cornstarch and sugar. Transfer 1/4 cup mixture to a large bowl; stir in 1 tablespoon oil and garlic. Add beef; toss to coat. Let stand 15 minutes., Heat a large skillet over medium-high heat. Add half of the beef mixture; stir-fry 1-2 minutes or until no longer pink. Remove from pan; repeat with remaining beef., In same pan, heat remaining oil over medium-high heat until hot. Add mushrooms and onion; cook and stir until mushrooms are tender. Add snow peas; cook 2-3 minutes longer or until crisp-tender., Stir remaining soy sauce mixture and add to pan. Bring to a boil; cook and stir 1-2 minutes or until sauce is thickened. Return beef to pan; heat through. Serve with rice if desired.