Preheat the oven to 350°F (with fan) or 400°F (without fan).
Place onions, potatoes and garlic in a baking tray and drizzle with olive onions, potatoes and garlic in a baking tray and drizzle with a generous glug of olive oil. Pour over stock, season well and toss so all vegetables are coated
Roast, give it a shake halfway through, for about 30 minutes, until vegetables are tender, and stock has all but evaporated.
Add the plums, sausages and rosemary.
Mix vinegar, sugar and stock together, pour over sausage mixture, season and toss to mix everything well.
Roast for a further 30 minutes, turning the sausages halfway through cooking time.
Increase heat for the last 10 minutes to brown the edges.