Preheat the oven to 220°C.
Sieve 75g flour into a bowl and make a well in the center.
Add the egg, salt, and pepper to the well.
Whisk in 75ml milk and 55ml water gradually until the batter is smooth.
Toss sliced onions with 5ml oil and sugar, then spread on a baking tray.
Arrange sausages in a roasting tin and roast for 10 minutes with the onions.
Remove sausages and roast onions for another 5 minutes until blackened at the edges.
Heat the roasting tin over medium heat on the hob until the fat is searing hot.
Pour the batter around the sausages and bake on the top shelf for 30 minutes.
Mix Worcestershire sauce and mustard powder into the vegetable stock.
Add onions, 5ml oil, and 10g flour to a pan and stir over medium heat.
Gradually whisk in the stock and simmer for 5 minutes.
Serve the puffed, crisp toad immediately with the onion gravy.