- Preheat broiler to low.
- Heat oil and butter in a large skillet until foamy. Add mushrooms and gently sauté until they begin to soften and brown, about 5 minutes. Remove from heat.
- Place mushrooms top side down in a single layer in a baking pan or on cookie sheet. Mound leftover dressing in the center of each cap. Sprinkle with parmesan cheese and broil until hot and bubbly, about 2 minutes.