Heat oven to 220C/200C fan/gas 7. Spread the sausages in a roasting tin and cook for 20-25 mins, turning, until golden and cooked through.
Meanwhile, fry the onions in a splash of oil over a high heat until almost softened and browning. Stir in the sweet chilli sauce and tomato purée with some seasoning, and cook for 1 min more.
Split the hot dog buns, add a spoonful of the sweet chilli onions, top with a sausage, then a spoonful more onions.