Start by preparing the pancake batter; sift the flour with a pinch of salt into a mixing bowl. Add the egg and half of the milk and whisk, starting slowly, until you have a smooth, thick batter. Whisk in the rest of the milk until you have the right consistency.
Add teaspoons of the melted butter to a small frying pan and heat over a medium heat. Fry the pancake batter, 2-3 tablespoons per pancake, making sure you tilt the pan to coat the surface as soon as you have added the batter. Keep the cooked pancakes warm on a plate lined with aluminium foil that is big enough to wrap over the pancakes and cover them loosely.
Preheat the oven to 200°C.
Heat the olive oil in a large frying pan set over a medium heat. Sweat the onions for a 6-8 minutes, stirring occasionally, until softened, then remove from the heat. Lay the pancakes on a flat surface and fill the centres with the sausage and onions, then roll up around the sides.
Arrange in baking dishes, side by side, then sprinkle the grated cheese on top. Bake for 15 minutes until the cheese has melted and the filling is hot. Remove from the oven and serve immediately.
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