Brisket Melt with Steak Sauce

Name: Brisket Melt with Steak Sauce
Ingredients: 2 tablespoons clarified butter, plus more for toasting; 1/2 Spanish onion, sliced; 1 cup sliced cremini mushrooms; Salt; 4 thick-cut slices rye bread; 8 slices mozzarella; 10 ounces sliced cooked brisket; 1/4 cup Steak Sauce, recipe follows; 2 tablespoons extra-virgin olive oil; 4 cloves garlic, ground; 2 shallots, ground; 1 portobello cap, ground; 1/2 cup Worcestershire sauce; 1/4 cup balsamic vinegar; 2 tablespoons ketchup; 2 tablespoons brown sugar; 1 teaspoon Dijon mustard; 1 teaspoon salt; 1/4 teaspoon chili powder; 1/8 teaspoon ground allspice; 2 tablespoons dark soy sauce; 1 tablespoon molasses