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.
