Cover potatoes with water in a medium saucepan. Bring to boil; simmer until tender, 25 to 30 minutes. Drain, cool slightly, and cut into slices. Place slices into a large heatproof bowl.
Fry the bacon in a medium saucepan over medium heat until almost crisp. Add the onion and cook until translucent, 5 to 10 minutes. Mix in the flour, sugar, mustard, salt and pepper. Stir in the vinegar, broth, and celery seeds. Cook, stirring often, until thickened.
Meanwhile, heat the knockwurst according to package directions; cut each into 6 to 8 diagonal slices. Add the knockwurst to the potatoes. Pour the sauce over and toss to coat. Sprinkle with the parsley and serve warm.