Rinse peas.
In a large saucepan simmer peas, bay leaf, and parsley sprigs in enough water to cover by 2 inches until tender, about 25 minutes.
Peas may be cooked 2 days in advance and kept covered and chilled.
Drain peas and discard bay leaf and parsley sprigs.
In a large bowl whisk together vinegar, mustard, horseradish, and salt and pepper to taste and add oil in a stream, whisking until dressing is emulsified.
Add peas, cabbage, carrots, scallion, garlic, minced parsley, and salt and pepper to taste and toss well.
Slaw may be made 1 day in advance and kept covered and chilled.
