Preheat the oven to 220C/425F/Gas 7.
Begin by making the batter, and to do this sieve the flour into a large bowl, holding the sieve up high to give the flour a good airing. Now, with the back of a spoon, make a well in the centre, break the egg into it and add some salt and pepper. Now, measure the milk and 2fl oz/55ml water in a measuring jug, then, using an electric hand whisk on a slow speed, begin to whisk the egg into the flour - as you whisk, the flour around the edges will slowly be incorporated. Then add the liquid gradually, stopping to scrape the flour into the mixture. Whisk until the batter is smooth. Now the batter is ready for use, and although it's been rumoured that batter left to stand is better, I have never found this, so just make it whenever it's convenient.
Now place the sliced onions in a bowl, add one teaspoon of the oil and the sugar and toss the onions around to get the lightest coating, then spread them on the baking tray. Next arrange the sausages in the roasting tin, then place the onions on a high shelf in the oven, with the sausages on a lower shelf, and set a timer for 10 minutes. When the timer goes off, remove the sausages from the oven but leave the onions in for a further 4-5 minutes - they need to be nicely blackened round the edges. When they are ready, remove them and leave to one side.
Now place the roasting tin containing the sausages over direct heat turned to medium and, if the sausages haven't released much fat, add the tablespoon of oil. When the tin is really hot and the oil is beginning to shimmer - it must be searing hot - quickly pour the batter in all around the sausages. Immediately return the roasting tin to the oven, this time on the highest shelf, and cook the whole thing for 30 minutes.
Now for the gravy. First add the Worcestershire sauce and mustard powder to the stock, then add the onions from the baking tray to a medium-sized pan. Now add the second teaspoon of oil, then, using a wooden spoon, stir in the plain flour. Stir all this together over a medium heat and then switch to a whisk, then gradually add the stock to the pan, whisking all the time, until it's all in. Then bring it up to simmering point and gently simmer for 5 minutes. Taste to check the seasoning, then pour into a warmed serving jug.
When the toad is ready, it should be puffed brown and crisp and the centre should look cooked and not too squidgy. Serve it immediately with the gravy, and it's absolutely wonderful with mashed potato.