1. In a large Dutch oven or heavy pot, heat 1 tablespoon oil over medium-high. Add sausages and cook until brown on all sides, about 8 minutes. Add onion and cook until softened, about 7 minutes. Add beer, potatoes, and 2 cups water; season with salt and pepper and press to submerge potatoes in cooking liquid. Bring to a boil; cover, reduce heat to medium, and cook until potatoes are tender, about 20 minutes.

2. Transfer sausages to a serving platter and keep warm. In a large bowl, stir together 1 tablespoon oil, vinegar, and parsley. With a slotted spoon, transfer potato mixture to dressing (reserve cooking liquid) and toss to combine.

3. Return pot to high heat; boil cooking liquid until reduced to 1 cup, about 12 minutes. Return sausages to pot and cook until heated through, 2 minutes. Place sausages and dressed potatoes on serving platter; drizzle half the sauce over top. Serve sausages and potatoes with remaining sauce alongside.