Step 1 Mince the mushrooms or process in a food processor until finely and evenly minced.
Step 2 Heat a large skillet over high heat until hot. Add the olive oil and butter and melt quickly, then stir in the mushrooms and salt. Cook, stirring frequently, until the liquid released from the mushrooms evaporates. Stir in the shallot and continue to cook just until the mushrooms begin to darken, another minute or so.
Step 3Stir in the wine, careful of the steam as it is very hot and could burn. Stir occasionally until the wine evaporates, 2 to 3 minutes, then continue to cook until the mushrooms are very dry and richly colored, 6 to 8 minutes. Remove from heat and spread the mushrooms out on a rimmed baking sheet to cool completely. This makes a scant cup of duxelles.