Heat a drizzle of oil in a heavy-bottomed, small frying pan – a cast-iron one is perfect. Add the onion and cook for a few mins until soft, then push them to one side of the pan. Add the sausages and fry for 5-10 mins, rolling them around in the pan until golden brown. Add the tomatoes, sugar and harissa, season well and add a splash of water. Cook for 5-10 mins, stirring now and then, until the tomatoes have cooked down to a thick, chunky sauce.