To make the batter, put the flour into a large bowl, season and make a well in the centre. Whisk the milk, mustard and egg together, then pour a third of the mix into the well. Gradually draw the flour into the milk to form a paste, then whisk in the rest of the milk to make a smooth batter.
Preheat the oven to 240° Celsius (220° for a fan oven), gas mark 9. Brown the sausages all over on a high heat for 5 mins to ensure the outsides are brown and crispy. Brush a Yorkshire pudding tray with 4 large compartments with oil, then place in the oven to heat; a hot tray and oil will make the batter mix rise better.
Cut the sausages in half with a pair of kitchen scissors and put into the Yorkshire pudding tray with the apple slices, then pour the batter over. Bake for 20 mins until risen, crisp and golden. Serve with mashed potato, peas and gravy.