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
Special equipment: Heavy extra-large wide ovenproof pot