Step 1: Preheat oven to 375°F (190°C) and grease a 9×9 inch baking dish.
Step 2: Boil potatoes in salted water for 15-20 minutes until tender, then drain.
Step 3: Mash potatoes with butter, milk, sour cream, garlic powder, pepper, and salt until smooth.
Step 4: Cook bacon in a skillet over medium heat until crispy, about 8-10 minutes, then remove and drain on paper towels.
Step 5: In the same skillet with bacon drippings, whisk in flour and cook for 1 minute to make a roux.
Step 6: Gradually whisk in beef broth until smooth, then simmer for 5 minutes until thickened to create gravy.
Step 7: Spread half the mashed potatoes in the baking dish, top with half the bacon and half of both cheeses.
Step 8: Pour half the gravy over the first layer, then repeat with remaining potatoes, bacon, cheese, and gravy.
Step 9: Bake for 20-25 minutes until bubbly and cheese is melted.
Step 10: Garnish with sliced green onions before serving.