Grill the sausages until cooked through and golden. While the sausages are cooking start on the onions.
Heat a little oil and butter in a heavy based saucepan and sauté the onions over a medium heat, until softened. This will take about 20 minutes.
Add the sugar and wine vinegar and continue to cook for another 2-3 minutes. Season to taste with a little salt.
Heat the milk with the butter. Mash the potatoes while they are still warm.
Add the milk, butter, mustard and a little salt and pepper and mix well. Serve with the sausages and caramelised onions.