Preheat oven (electric cooker: 200 °C/circulating air: 180 °C/gas: see manufacturer). Bake the hot dog buns in the oven for approx. 5 minutes and cut lengthwise.
Warm the sausages in hot water. Drain the beetroot and cut into fine cubes. Drain cucumber salad.
Fill each roll with 1 sausage, beetroot and cucumber salad. Spread remoulade, ketchup and roasted onions on top, serve immediately.