Whisk together the flour, mustard powder, eggs and milk with a good pinch of salt until smooth. Leave to rest for at least 30 mins.
Heat 1 tbsp of the oil in a small pan. Add the onions with a pinch of salt and cook for 10-15 mins until really soft. Meanwhile, make up the stuffing following pack instructions, adding the sage. Scoop out a quarter of the onions and add these to the stuffing mixture, too - leave the rest in the pan for the gravy. Heat oven to 230C/210C fan/gas 8.
Shape the stuffing into 8 walnut-sized balls. Pour the remaining oil into a large roasting tin or dish and brush it all over the bottom and sides. Add the sausages and stuffing balls, and cook for 15 mins.
Remove the tin from the oven, loosen the sausages and stuffing from the bottom of the tin if they are sticking, then pour over the rested batter. Return the tin to the oven on a middle-high shelf, but give it space to rise. Bake for 35-40 mins until puffed and golden - don’t be tempted to open the oven door any earlier or the batter might sink.
While the toad-in-the-hole is cooking, make the gravy. Add the flour, onion chutney and Marmite to the onion pan and mix to a paste. Cook for 2 mins until bubbling, then pour in the stock, bit by bit, stirring continuously so it doesn’t go lumpy. Bubble for 5 mins and season if needed. Keep warm until ready to serve.