In a large saucepan, cook mushrooms, shallot, and garlic in 1 tablespoon butter over medium-low heat about 5 minutes or until tender. Add brandy and vinegar, stirring to loosen browned bits from bottom. Bring to boiling; reduce heat. Boil gently, uncovered, about 7 to 9 minutes or until sauce is reduced to 1/2 cup.
In a small bowl, combine 2 tablespoons of the demiglace and the cornstarch. Stir until well combined. Add to mushroom mixture along with remaining demiglace, the mustard, parsley, and remaining 1 tablespoon butter. Cook and stir until thickened and bubbly. Cook and stir 2 minutes more. Serve over grilled steak. Makes 2 cups sauce (ten 3-tablespoon servings).