Heat the oil in a medium frying pan (one the sausages will fit in fairly snugly). Add the sausages and fry for 8-10 minutes, turning them often. Turn the grill to high. Splash a couple of tablespoons of red wine into the pan if you are using it, then pour in the stock and stir in the caramelised red onions. Allow the mixture to bubble for 3-4 minutes, so it thickens a little and turns into a rich gravy. Remove from the heat.
Spread the potatoes so they roughly cover the sausages and gravy. Put the frying pan under the grill for about 8 minutes until the potatoes turn crisp and golden. Serve while bubbling and hot – there is no need to add seasoning.