Duxelles

Duxelles is a French cuisine term that refers to a mince of mushrooms, onions, herbs, and black pepper, sautéed in butter and reduced to a paste. Cream is sometimes used, and some recipes add a dash of madeira or sherry.
It is a basic preparation used in stuffings and sauces or as a garnish. It can also be filled into a pocket of raw pastry and baked as a savory tart.
The flavor depends on the mushrooms used. For example, wild porcini mushrooms have a much stronger flavor than white or brown mushrooms.
Duxelles is said to have been created by the 17th-century French chef François [Pierre La Varenne] and to have been named after his employer, Nicolas Chalon [du Blé, marquis d'Uxelles], maréchal de France.
Some classical cookbooks call for dehydrated mushrooms. According to Auguste Escoffier, dehydration enhances flavor and prevents water vapor from building up pressure that could cause a pastry to crack or even explode.