Brisket Braised in Porter

Name: Brisket Braised in Porter
Ingredients: 1 tablespoon coarse kosher salt; 2 teaspoons freshly ground black pepper; 2 teaspoons dry mustard (such as Colman's); 2 teaspoons chopped fresh sage; 2 teaspoons chopped fresh thyme; 1 6-pound flat-cut brisket, trimmed but with some fat still attached; 2 tablespoons rendered bacon fat or olive oil; 4 cups (or more) low-salt chicken broth, divided; 1 12-ounce bottle porter or stout; 6 whole pitted prunes; 4 bay leaves; 2 teaspoons (packed) dark brown sugar; 6 cups thinly sliced onions (2 1/2 pounds); 8 whole garlic cloves, peeled; 1 pound mushrooms, sliced; 1 pound medium carrots, peeled, cut crosswise into 1 1/2-inch lengths; 2 tablespoons whole grain Dijon mustard; 1 tablespoon (or more) malt vinegar; Heavy extra-large wide ovenproof pot