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Tip the flour into a large jug or bowl. Crack in the eggs, one at a time, whisking as you do. Pour in the milk and continue whisking until you have a smooth, lump-free batter. Add the mustard, herbs and some seasoning, then cover and set aside for 2 hrs, or chill overnight.
Heat oven to 220C/200C fan/gas 7. Heat the oil in a large metal roasting tin or enamel baking dish, roughly 30 x 22cm, with high-ish sides. Wrap a rasher of bacon around each sausage and pop them in the dish with the onion wedges. Place on the middle shelf of the oven (make sure the top shelf is removed to allow space for the batter to rise later). Cook for 15-20 mins until the sausages and bacon are browning and sizzling.
Meanwhile, make the gravy. Drizzle a little oil into a saucepan. Add the sliced onions and cook slowly for 15 mins. Stir every now and then until they are soft and caramelised. Add the sugar and balsamic vinegar, and cook for 5 mins more until sticky.
The batter should be the consistency of double cream - if it has become a little thick, add a splash of cold water. Take the batter to the oven, open the door, carefully pull out the shelf and quickly pour the batter around the sausages. Close the oven and do not open it again for at least 25 mins.
To finish the gravy, stir the flour into the onions to make a paste, cooking for 1-2 mins. Continue stirring while you splash in the wine, a little at a time, until you have a smooth, thick sauce. Increase the heat and bubble for a few mins until reduced by about half. Add the stock and some seasoning, and continue bubbling for 10 mins while the toad cooks.
By now the toad should be puffed up and deep golden brown. If not, continue cooking - but don't open the oven as it will sink. Serve scattered with the crisp herbs (if using), with the gravy, green veg, and mash, if you like.