This is from Martha Stewart Food. My husband and I eat it as a main dish, but it would make a nice appetizer also if you used smaller mushroom caps.
- Preheat oven to 450.
- Place mushroom caps, gill sides up, on a rimmed baking sheet.
- Drizzle with oil and vinegar, then season with salt and pepper.
- Bake until caps are just tender, approximately 15 minutes.
- Top with leeks, spinach and cheese.
- Bake until spinach begins to wilt and cheese starts to brown.