To make this recipe in a cast-iron skillet, preheat oven to 425°F. Prepare recipe as directed except use a 12-inch cast-iron skillet in step 1. Stir tuna into mushroom mixture in step 3 and top with cheese and potato nuggets. Bake 20 to 25 minutes or until potatoes are brown and center is bubbly. Let stand 10 minutes before serving. Sprinkle with green onions.