Melt butter in a saucepan over low heat and add garlic and sliced leek. Slowly
cook sliced leek, stirring often, until golden and softened, about 8 to 10
Stir in the flour and season to taste with salt, pepper and nutmeg.
until the flour changes color just slightly.
Remove garlic clove and then gradually add the milk, stirring
time. When sauce is smooth and thick, pour in cream. Bring back
boil, lower the heat and cook for 5 minutes.
Add Gruyere and cook, stirring, until cheese has melted. Remove
and cool slightly before serving. This sauce keeps well refrigerated
up to four days.
Recipe source "The New Pasta Cookbook"