Preheat the oven to 425 F. To make your batter, whisk eggs, water, and flour together in a big bowl until lump free. Slowly add milk, mayonnaise, salt, and pepper and whisk until well combined. Place in the fridge until required.
Stir together the oil and sausages to a casserole dish in a uniform pattern. Place the dish in the oven and cook for 10 minutes.
After 10 minutes, remove the dish from the oven, take the batter out of the fridge, and quickly pour it around the sausages, returning the dish to the oven. Bake for another 25-30 minutes. Do not open the oven until at least 25 minutes has passed to reduce the batter deflating. Serve immediately when crisp and warm!