Mince the white leek, the onion and the mushrooms.
Brown the leek and the onion over low heat with the butter for 5 minutes.
Add the mushrooms and the chicken broth then leave to simmer for 20 minutes.
Dissolve the cornstarch in the cold liquid cream.
Add the preparation to the velouté.
Thicken for 1 minute over low heat, stirring.
You can taste!