Suprême sauce is one of the classic "small sauces" of French cuisine, that is, one made by combining a basic or mother sauce with extra ingredients.
Traditionally, this sauce is made from a velouté sauce (a roux sauce made with a meat stock - in the case of suprême, a chicken stock is usually preferred), reduced with heavy cream or crème fraîche, and then strained through a fine sieve.
A light squeeze of lemon juice is commonly added. In many cases, chefs also choose to add finely-chopped and lightly sautéed mushrooms to the dish.
It is possible to make a similar sauce to pass for sauce suprême by taking béchamel sauce, with a poultry stock and butter.
The sauce is alternatively made by placing three-quarters of a pint of white sauce into a saucepan, and when it is nearly boiling, adding half a cup of concentrated fowl stock. It should then be reduced until the sauce is quite thick, passed through a chinois strainer into a bain-marie and have added two tablespoons of cream.
Recipe for Suprême Sauce Link 1 Link 2 Link 3