Preheat your oven to 200°C. Toss the potatoes and red onion wedges in a large roasting tin with olive oil, salt, and pepper. Spread them out so they have space to crisp.
Place your sausages on top of the veg. Roast for 25 minutes.
Whisk your balsamic, mustard, honey, and rosemary together.
Remove the tray and coat everything generously with the glaze. Turn the sausages over so they brown evenly on both sides.
Roast for a final 15–20 minutes until the sausages are dark, glossy, and cooked through, and the onions have turned into a sweet, sticky relish.