Preheat oven to 400° F. Line Sheet Pan with Sheet Pan Liner.
Remove mushroom stems and use a spoon to scrape out and discard gills. Place mushroom caps in Multipurpose Steamer. Drizzle with oil and sprinkle with seasoning; gently toss to coat. Cover and microwave on high 4 min.
Meanwhile, dice pepper.
When mushrooms are cooked, place on pan with underside of caps facing up. Divide sauce, cheese, and pepper between caps. Bake 8–10 min, or until cheese is bubbly.