- Finely dice the onion and mushrooms. Melt the butter in a pan and add the onion. Add the mushrooms and gently sauté for 5 minutes.
- Add the flour and mix until the mixture turns a golden colour.
- Remove the pan from the heat and gradually add the milk, stirring all the time.
- When the mixture has a uniform consistency, add the stock and put the pan back on the heat to thicken it.
- Add the cheese.
- Leave the mixture to cool for 2 hours.