Heat the oil in a frying pan and gently cook the sausages over a medium-high heat for 10-12 mins, turning them until browned on all sides. Transfer to a plate, then put the half of the butter in the pan and heat until sizzling. Tip in the sliced onions, stirring them into the butter, then sprinkle over the sugar and fry for 8-10 mins until the onions are golden. Scatter over the flour and stir to make a paste, then add the tomato purée. Cook for a minute, then add the vinegar, pour in the stock and bring to the boil. Turn down to a simmer.

Tip in any juices from the sausages, then cut the sausages into thick slices and add to the pan. Simmer everything together for 5 mins until you have a glossy onion gravy. Tip the mixture into a baking dish.

To make the mash, tip the peeled and cut potatoes into a pan of cold salted water and bring to the boil, then turn down and simmer for 10-12 mins or until just cooked (the tip of a knife should slide in easily). Drain and leave for a minute. Pour the milk into the pan and bring to a simmer, then tip in the drained potatoes and remaining butter, and mash thoroughly. Season to taste with salt and pepper. Preheat the oven to 200C/180C fan/gas 6.

Top the sausages and gravy with the mash, starting from the edge of the dish and working your way into the middle. Use a fork to scrape lines in the topping and scatter over the cheese, if using. Bake the pie on a tray for 35-40 mins until the top has browned. Leave to cool for 5 mins before scooping straight from the baking dish at the table and serving with peas, if you like.