Cut daikon into 2-inch-long fine julienne strips, a mandolin os your friend here! Rinse large bowl with hot water, dry it, and in it whisk mustard with 3 Tbsp hot water. Add oil in slow stream, whisking until dressing is emulsified, and whisk in vinegar and salt and pepper to taste. Add daikon strips, scallions and parsley (reserving a small amount of each for garnish) and toss mixture well. Serve on a bed of lettuce, garnished with additional parsley and scallions.