Step 1

Put the mushrooms and shallots in a food processor fitted with the chopping blade. Pulse until finely chopped. The mushrooms and shallots should be in about 1/8-inch pieces—don’t overprocess.

Step 2

Melt the butter in a 10-inch skillet over medium heat. Add the mushroom mixture, salt, and pepper and cook, stirring. When the mushrooms begin to release their moisture, turn the heat to low and cook, stirring occasionally, until the liquid has evaporated and the duxelles look dry, 20 to 25 minutes. Let cool.