How to Make It
In a large pot, combine the corned beef and its seasonings with the beer, onion, garlic, bay leaves and 12 cups of water and bring to a boil over high heat. Cover and simmer over low heat until the beef is tender, about 2 hours and 30 minutes.
Meanwhile, in a medium saucepan, combine the potatoes with the vinegar, mustard seeds, sugar, salt and 3 cups of water and bring to a boil. Simmer over moderate heat until the potatoes are tender, 12 to 15 minutes. Transfer to a large heatproof bowl and let cool completely in the liquid. Stir in the cabbage and refrigerate for 1 hour, stirring occasionally. Drain well, transfer to a serving bowl and stir in the parsley.
Transfer the corned beef to a carving board and let rest for 15 minutes, then slice against the grain. Serve alongside the salad, passing the mixed mustards at the table.