Preheat the oven to 220C/Gas 7 and place your casserole dish inside and allow to get super hot.
Make the Yorkshire pudding batter by whisking the flour, eggs and milk together, allow to stand for 20/30 minutes before using.
Pop a frying pan onto a medium high heat and gently fry the sausages until they become golden and remove the pan from the heat
When your casserole dish has been in the hot oven for 10 minutes remove and add a splash of olive oil then pour in your YP batter, be quite liberal with this as you want a nice pudding bottom, then place your browned sausages into the middle, pop back into the oven and allow to bake for 10/15 minutes or until the pudding has risen and is cooked. Remove and serve with your onion gravy