Cut the daikon into 2-inch-long fine julienne strips or grate it coarse. Rinse a large bowl with hot water, dry it, and in it whisk the mustard with 3 tablespoons hot water. Add the oil in a slow stream, whisking until the dressing is emulsified, and whisk in the vinegar and salt and pepper to taste. Add the daikon strips and the parsley and toss the mixture well.