I use pancake batter recipe, use 1/3 water and 2/3 milk. Add 2tsps of olive oil to mixture and leave in fridge the night before. Very hot oil in dish, I find coconut oil best as you need less. Add sausages then pour mix around them. Put in oven Do NOT open oven to check
I always use lard or dripping, heated until smoking before adding the batter. I also leave my batter to sit for as long as possible, giving a quick whisk immediately before using.
The quality of the sausages makes a big difference but I am sure you have your favourites.
One of the best I have ever eaten, I do like to colour the sausages in a frying pan first. Nothing worse than insipid looking cooked sausages. Nice use of mustard too. Spoil yourself:
https://www.bbc.co.uk/food/recipes/toadinthehole_3354
Put raw sausages in a metallic deep tray and cook at 225c turning once for about 15 mins. They will render fat which you can then add your batter mix into the tin. I don't eat it as it's really unhealthy but make it for my dad.