1. Preheat the oven to 230C/450F/gas mark 8. Divide the oil among 6 individual Yorkshire pudding tins and place on a baking tray at the top of the oven for 5 minutes until sizzling.
2. Meanwhile, slit the sausage skins and peel them away from the sausagemeat. Roll each sausage into 3 small balls and place 3 in each tin. Return to the oven for 5 minutes.
3. Divide the batter among the tins and return to the top of the oven for about 25 minutes until risen and golden.
4. While they are cooking, make the gravy. Heat the oil and butter in a large saucepan and cook the onions over a medium heat for about 15 minutes, stirring frequently until they are well browned and softened.
5. Stir in the flour and mustard, then slowly add the stock, stirring all the time. Bring to the boil, cover and simmer for 15 minutes until the onions are tender and the gravy is lightly thickened.
6. Serve with mashed potatoes and a green vegetable such as cabbage or peas.