In a saucepan filled with lightly salted boiling water, blanch leeks 6 minutes or so.
In a frying pan, melt butter.
Cook chopped onion 1 minute.
Fold in cream.
Continue cooking 2 minutes.
Meanwhile, remove crust from Brie.
Fold cheese into cream sauce.
Season with nutmeg, salt and pepper.
While stirring, continue cooking until Brie has completely melted.
Reheat leeks in sauce.