Preheat the oven to 220C/200C fan/Gas 7. Pour the oil into a medium roasting tin and place in the oven to heat up.
Whisk the flour, eggs, milk and salt together in a bowl until completely smooth. The batter should be the consistency of single cream.
Leave the batter to rest while the tin heats up. The oil needs to be shimmering hot before anything goes in.
Carefully add the sausages to the hot tin, turning them to coat in the oil. Return to the oven for 5 minutes to brown slightly.
Pull the tin out and pour the batter around and over the sausages. Get it back in the oven quickly.
Bake for 25-30 minutes without opening the oven door, until the batter is well risen, golden and crisp at the edges.
Serve immediately with mashed potato, green vegetables and onion gravy.