Preheat oven to fan 180C/conventional 200C/gas 6. Put a generous half-teaspoonful of sunflower oil in the bottom of each cup of a deep 12-hole mini muffin pan set on a baking tray.
Pour the milk into a jug, add the egg, flour and a pinch of salt and whisk everything together with a hand blender or wire whisk until a smooth batter forms.
Put the muffin pan in the oven for a minute or so until the oil is very hot. Carefully remove from the oven and quickly fill the cups just under two-thirds full (this may not seem enough batter, but they puff up in the oven). Drop a sausage into each cup and bake for 20-25 minutes until golden brown, well risen and crisp. Serve with ketchup.