In a bowl, add the flour and eggs and whisk to combine. Pour in the milk and whisk until smooth — some lumps are okay. Store in the fridge for 1 hour up to 24 hours; for best results, rest overnight.
Preheat the oven to 220°C (430°F).
Add the vegetable oil and sausages to a large baking dish or roasting pan with high sides. Bake for 15 minutes, flipping once, until browned all over.
Remove the batter from the fridge and season to taste with salt and pepper.
Remove the baking dish from the oven and place on a wire rack or heatproof surface. Pour over the cold batter, making sure the sausages are spread evenly. Immediately return to the oven and bake for 22–25 minutes, until fluffy, crispy, golden, and the sausages reach an internal temperature of 71–74°C (160–165°F). Serve with onion gravy and dig in.
Tie the thyme, rosemary, and sage into a bouquet garni with kitchen string. Melt the butter in a large saucepan over medium heat. Add the onion and sugar, then cook, stirring often, for 30 minutes, until caramelised.
Increase the heat to medium-high. Add the wine and bouquet garni, then bring to a boil. Reduce the heat to medium and simmer for 5 minutes to reduce the wine. Add the beef stock, Worcestershire sauce, and mustard, and season to taste. Reduce the heat to low and simmer for 15–20 minutes, until it coats the back of a spoon. Remove and discard the bouquet garni.
Whisking constantly, add the balsamic vinegar and cornflour slurry. Simmer, whisking constantly, for another minute until thickened. Add the cold butter and whisk to incorporate. Season to taste, adding more mustard if desired.