Meanwhile, caramelize onions. Halve and slice onions.
Coat 12-inch skillet with cooking spray. Over medium heat, cook onions in oil for 15 to 20 minutes, stirring occasionally, or until soft and golden. Stir in garlic powder, per taste.
Place potatoes on a rimmed cookie sheet.
Add caramelized onions on top of potatoes.
Sprinkle lots of grated Cheddar cheese over the top.
Bake for 5 minutes at 350 degrees F or just until the cheese is melted.
Garnish with dollops of sour cream and green onions.