Place mushrooms and onion in a 3-qt. slow cooker. Sprinkle beef with salt and pepper. In a large skillet, heat 1 teaspoon oil over medium-high heat; brown meat in batches, adding more oil as needed. Transfer meat to slow cooker., Add wine to skillet, stirring to loosen browned bits from pan. Stir in broth and Worcestershire sauce; pour over meat. Cook, covered, on low 6-8 hours or until meat is tender., In a small bowl, mix cornstarch and cold water until smooth; gradually stir into slow cooker. Cook, covered, on high until gravy is thickened, 15-30 minutes. Serve with mashed potatoes.