This is a classic dish in French cuisine. However, since this is under the terms page, I am only going to explain what it is here. Again, classic dish in French cuisine; it is a preparation consisting of a mixture of minced mushrooms, onions, herbs (usually including thyme and or parsley), and black pepper. The mixture is then sauteed in butter with the goal of reducing it down almost to a paste.
Of course, you could have quite a few variations based upon this simple recipe. You could reduce it sherry, replace the onions with shallots, and more. The dish is French in origin and, according to the James Beard Foundation, was created by the chef François Pierre la Varenne. Like other dishes, this one was named after is patron, the Marquis d’Uxelles.