Preheat grill or grill pan.
Clean mushrooms and lay with gills up in a shallow baking pan. In a bowl combine olive oil, butter, red pepper flakes, garlic, wine and zest. Whisk to combine and pour over mushrooms. Marinate mushrooms at room temperature for 20 minutes, turning once. Pat mushrooms dry and place gill side up in grill pan. Grill slowly for about 20 minutes, then turn and grill for about 5 to 10 minutes more. Slice and serve with polenta. Garnish with chopped parsley.