To make the dressing: In a small frying pan over medium heat, add 2 tablespoons of olive and sauté the onion until just soft, about 5 minutes. Add the lager, the vinegar, and sugar and boil for 5 minutes. Transfer to a food processor and add the mustard. With the motor running, slowly pour in the remaining 4 tbsp. olive oil. Season to taste with salt and pepper.
To make the salad: In a large skillet cook the sausage over medium heat for 5-7 minutes, transfer to a large salad bowl and keep warm.
Boil the potatoes in salted water until tender, about 20-25 minutes. Drain and slice into ¼ -inch rounds. While the potatoes are still warm, gently mix with the sausage, onions, parsley and dressing. Season to taste with salt and pepper. Garnish with chopped chives.