Preheat the roasting oven.
Sift the flour into a bowl with a pinch of salt. Break the egg into the bowl. Whisk in the milk and the mustard into a smooth batter. Set aside for 5 minutes to rest.
In the meantime, on the boiling stovetop, sauté the sausages and onions in an ovenproof or aga iron pan. Colour on all sides for 4-6 minutes until browned.
Add the extra oil or fat to the pan and heat it up until piping hot. This will help rise the batter like a Yorkshire pudding.
Heat the oil for 2 minutes or until piping hot. Carefully pour in the batter. Transfer the pan in the oven and bake for 20-25 minutes until golden and just firm.
Remove from the oven and serve immediately with a red wine onion gravy.