Duxelles is a finely chopped mixture  of mushrooms, onions or shallots,  

and often herbs such as thyme or parsley,  sautéed in butter, and reduced to a paste.

It is a basic preparation used in  classic French cuisine. Sometimes,  

the cream is used and is cooked  down to a thick mixture.

This mushroom paste is often used as  a stuffing or filling in many dishes,  

such as beef Wellington, where it's spread  over the beef before it's wrapped in pastry.

It can also be used as a flavor base for  sauces, or to stuff into pastries, poultry,  

or vegetables. It's an excellent way to add an  intense mushroom flavor to a variety of dishes.