1: Make the Yorkshire pudding batter the day before by whisking the milk and eggs together, then slowly incorporating the flour. Be careful not to mix too vigorously, as this will stretch the gluten in the flour. Put the mixture in a plastic container and let it stand in the fridge for 24 hours.
2: The next day, wrap the sausages neatly in the bacon and refrigerate until ready to cook. Preheat the oven to 180-c.
3: To make the gravy, cut the onions in half through the equator and sear them middle side down in a pan until the rings have gone black. Then, place the onions on a baking tray lined with tin foil. Sprinkle on the demerara sugar and the thyme and drizzle with a little oil. Seal the tin foil up into a bag, place into the oven and roast for 3.5-4 hours until the onions are really caramelised and soft.
4: When they are done, scoop out the onion flesh and whizz in a food processor until a brown rich pulp is formed.
5: Melt the butter in a saucepan then and the flour to make a roux-style base. Cook for 1-2 minutes. Pour in the dark ale and the onion pulp, add a good pinch of salt and cook on a low heat for 30-40 minutes until it has reduced by about a quarter. Taste, and season if required.
6: Heat oven to 220-c (200-c fan assisted).
7: Put the sausages in a roasting tray, then place in the oven for 15 minutes to colour.
8: Pour the Yorkshire pudding batter mix into the tray with the sausages. Bake for a further 40 minutes.
9: Be careful not to open the oven door during this time as the Yorkshire pudding will sink! Serve the toad in the hole with the onion gravy and English mustard mash.