Preheat the oven to 230°C.
Prepare the Yorkshire pudding batter by sifting the flour into a bowl with some seasoning, then whisk the egg in until incorporated. Whisk in the flour and water until you have a smooth batter. Strain into a jug and set to one side.
Melt the butter with the olive oil in a large frying pan set over a medium heat. Sweat the sliced onion for 8-10 minutes, stirring occasionally. Add the caster sugar and a little seasoning and continue to cook for a further 8-10 minutes, stirring occasionally, until golden and starting to caramelise. At the same time, divide the sunflower oil for the Yorkshire pudding evenly between a 4-hole Yorkshire pudding pan.
Heat the oil in the oven for 10 minutes until smoking hot. Remove the pudding pan and carefully pour the Yorkshire pudding batter into the oil, filling it up to the top of the moulds. Return to the oven and bake for 8-10 minutes until golden, risen and crispy. Remove the onions from the heat at this point. Preheat the grill to hot. Grill the chipolatas for 8-10 minutes, turning occasionally to ensure even cooking all over.
Remove the Yorkshire puddings when ready and reheat the onions over a medium heat. Remove the chipolatas when they are ready and cut each one into three. Arrange the Yorkshire puddings on 4 serving plates and spoon the onions and the chipolata pieces on top. Serve immediately.
For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home and view the full Food safety at home video.