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.
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.
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.
Italian sausages are delicious in this recipe, but other sausages, such as chicken or turkey, also work well.