Combine mustard, olive oil, vinegar, beef stock, sugar, salt and pepper into a bowl and whisk well. This is your dressing. Peel and slice the precooked knockwurst into strips. Cut onion into thin rings. Dice the boiled potato. Slice the pickle thinly. Place knockwurst, onion, potato, and pickle in large bowl. Pour dressing over salad and toss gently. Refrigerate 2 hours. Peel and thinly slice egg; mix with salad. Serve on lettuce leaves with a side of rye bread.