Preheat your oven to 200°C/180°C(fan)/400°F/Gas 6.
Put 8 Pork sausages, 1 Red onion and 2 tablespoon Vegetable oil into a baking dish. Mix well and cook for 15 minutes.
In a large mixing bowl, sift 150 g Plain flour, add 1 pinch Sea salt and crack in 3 Eggs.
Whisk together to form a thick paste.
Gradually add 220 ml Whole milk and 2 tablespoon Water, continuing to whisk, until a smooth batter is formed.
Remove the sausages from the oven and pour the batter around them.
Return to the oven for 30 minutes. Do not open the oven until the cooking has finished.
Notes
The most important tip here is to not open the door, certainly not for the first 20 minutes, because the batter will sink if you do.
Like when making Yorkshire puddings, it's best to use a metal tin to make them rather than a ceramic dish, as it gets hotter which is key to them getting crispy.
Some say that adding a little mayo to your Yorkshire pudding batter can help them rise more, but it sounds a bit odd so I have never tried it myself.
You can add some sprigs of thyme at the same time as the onion for extra flavour. Just take them out before you pour the batter in.