Cook bacon in a 9-inch cast iron skillet over medium heat until crisp. Remove bacon to drain on paper towels; set aside. Scrape the bottom of the pan if needed with a metal spatula. Pour off all but 1 tablespoon of the drippings. Add potatoes to skillet; cook 2 minutes, stirring occasionally. Preheat broiler.
Beat eggs in a medium bowl. Stir in milk, salt and 1 cup of cheese. Add egg mixture to skillet; stir to combine. Cover; simmer until eggs are set on the bottom, 6 to 8 minutes. (Center will be wet.)
Uncover; top with reserved half of the bacon, remaining cheese, then the rest of the bacon. Transfer skillet to broiler; broil 5 to 6 inches from heat for 2 to 3 minutes or until eggs are set and cheese is melted. Garnish with chopped green onions. Cut into 6 wedges.