In a large dutch oven, heat 1 tbsp of oil over medium-high heat. Add sausages and cook until browned on all sides, about 4 minutes Add onion and cook until softened, about 7 minutes.

Add beer, potatoes and broth. Season with salt and pepper. Make sure potatoes are fully submerged. Bring to a boil; then cover, reduce heat to medium, and cook until potatoes are tender, about 20 minutes.

Transfer sausages to a platter and keep warm with tented foil. In a large bowl, bomcine remaining tbsp of oil, vinegar, mustard and parsley, whisk until fully emulsified. Use a slotted spoon to transfer the potatoes to the bowl with dressing, but reserve the cooking liquid, then toss the potatoes in the dressing until completely coated.

Return the pot to high heat and boil cooking liquid until reduced to 1 cup, about 10 minutes. Add potatoes to the serving platter with sausages and drizzle ½ the sauce over the top. Reserve remaining sauce to serve on the side.