Melt the butter, olive oil, lemon juice, garlic & pepper together in a
small pot or on the grill.Slice any mushrooms that are over 3
inches in diameter into strips. Brush the mushrooms generously with
the melted mixture.On a covered grill, over medium-hot coals, grill
the mushrooms upside down for about 8 minutes. Trun & grill the tops
for another 8 minutes. The mushrooms should be very tender & well
browned.
Kelly McCune, "Vegetables on the Grill"