On a hot griddle, add the clarified butter, and then add the onions. Cook until just tender, and add the mushrooms. Season with salt, and cook until caramalized. Set aside to cool.
To assemble the sandwich, top one slice of the bread with 2 slices of mozzarella, 5 ounces sliced brisket, 2 tablespoons of the Steak Sauce, half of the caramelized onions and mushrooms and 2 more slices of mozzarella, then top with another slice of bread. Repeat for the remaining sandwich. Ladle some clarified butter on the griddle and place the sandwiches in the butter. Cook the sandwiches on both sides until the bread is toasted, the meat is warm and the cheese melts.
Heat the oil in a medium saucepan set over medium-high heat. Saute the garlic, shallots and portobello for 3 minutes. Add in the Worcestershire, vinegar, ketchup, sugar, mustard, salt, chili powder and allspice. Simmer for 15 minutes. Add 2 cups water, and then cook for 10 minutes. Add in the soy sauce and molasses, and cook for 5 minutes. Blend until smooth and cool.
When blending hot liquid, first let it cool for five minutes or so, then transfer it to a blender, filling only halfway. Put the lid on, leaving one corner open. Cover the lid with a kitchen towel to catch splatters, and pulse until smooth.