Lentil Salad with Knockwurst

Name: Lentil Salad with Knockwurst
Ingredients: 5 flat-leaf parsley sprigs, stems reserved and leaves coarsely chopped; 1 leek, green part only; 1 bay leaf; 1 pound French green lentils (2 cups), rinsed and picked over; 1 large yellow onion, halved lengthwise; 1 celery rib, halved crosswise; 1 large carrot, halved crosswise; 2 quarts cold water; 1 pound cooked knockwurst, casing removed, sausage sliced 1/4 inch thick; 1 small red onion—thinly sliced crosswise, rinsed in cold water and patted dry; 1 garlic clove, minced; 2 tablespoons red wine vinegar; 1 1/2 tablespoons Dijon mustard; Salt and freshly ground white pepper; 3 tablespoons extra-virgin olive oil