How to Make It
Trim and discard most of the fat from the surface of the brisket. Rinse meat thoroughly under cool running water, massaging to release the salty brine.
In a 5-quart or larger electric slow-cooker, combine onions, carrots, potatoes, peppercorns, allspice, cloves, and bay leaf. Lay meat, fattiest side up, on vegetables. Add the ale. Cover and cook until brisket is very tender when pierced, 8 to 9 hours on low, 6 to 7 hours on high. If possible, turn meat over about halfway through cooking.
With 1 or 2 slotted spoons, transfer brisket, fattiest side up, to a 10- by 15-inch pan. Using hot mitts, drain juice from cooker into a 5- to 6-quart pan; turn heat in slow-cooker to low to keep vegetables warm. Add cabbage to juice and bring to a boil over high heat. Cover and cook until cabbage is brighter green and barely tender when pierced, 5 to 7 minutes. Meanwhile, spread fatty side of brisket with 1/2 cup mustard and broil about 6 inches from heat until surface sizzles, about 3 minutes.
Transfer corned beef, mustard side up, to a large platter, spoon vegetable mixture and cabbage around meat. Pour cooking juices into a pitcher. Slice brisket across the grain. Serve meat and vegetables with juices and additional mustard to add to taste.