Meanwhile, in 12-inch nonstick skillet, melt butter over medium heat. Cook onion, garlic and mushrooms in butter 8 to 10 minutes, stirring occasionally, until mushrooms are completely tender and beginning to brown. Stir in sherry. In small bowl, mix cornstarch and water; stir cornstarch mixture into mushroom mixture. Stir in thyme, pepper and beef consommé; cook and stir about 2 minutes or until slightly thickened and bubbly. Stir in mustard and tomato paste until well blended. Heat just until hot. Spoon sauce over sliced beef.