Preheat the oven to 220°C.
Place the flour and salt into a large bowl and create a well in the center.
Whisk the eggs with the milk and mustard in a separate bowl.
Pour the egg mixture into the flour and whisk into a thick batter.
Rest the batter for a few minutes while preparing the meat.
Brown the sausage in a frying pan with a drizzle of olive oil.
Heat 2 tbsp of vegetable oil in a gratin dish in the oven until very hot.
Place the sausage into the hot oil and pour the batter over it.
Bake for 15 minutes until the batter is puffed and golden-brown.
Serve immediately.