Heat a 12-inch skillet over medium-high heat and add the mushrooms to the dry pan.
Cook for 6 to 8 minutes, stirring occasionally, until the released liquid evaporates.
Add the butter, oil, onion, salt and pepper and spread everything into an even layer.
Cook for 10 to 12 minutes, stirring every few minutes, until the mushrooms and onions have deep brown edges.
Add the Worcestershire sauce and toss for 1 minute until the vegetables look glazed and the pan is nearly dry.
Serve hot without covering, which would soften the browned surfaces.