Step 1

Heat the butter in a large frying pan. Add the onion and sprinkle with the sugar, then cook gently, stirring occasionally, for 10 to 15 minutes or until the onion is soft and lightly caramelised.

Step 2

Sprinkle the flour over the onion, stir and cook for 1 minute. Gradually stir in the stock. Bring to the boil, then reduce the heat and leave to simmer gently until thickened.