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.