Finely chopped mushrooms and shallots, sautéed in butter, cooked down to a paste and used as a basis for sauces. It is sometimes mixed with cream or may have white wine and/or garlic added to the mixture. Heston Blumenthal gives some tips about how to use duxelle. He suggests mixing it with spinach and herbs to serve with pasta, that it is delicious with tomatoes, or spread on toast with a little mustard and parsley. Turnips, and I would have thought that butternut squash, can be stuffed with duxelle mixed with the flesh.