Heat 1/4 teaspoon oil in a large skillet over medium-high heat. Add beef and onions; saute until beef is mostly browned, 7 to 9 minutes. Drain 90% of the fat.
Cut open the bag of hash browns; microwave until mostly defrosted, about 4 minutes.
Heat butter and the remaining 1/4 teaspoon oil in the skillet over medium heat. Add hash browns to the beef-onion mixture. Cook and stir until lightly browned, about 3 minutes. Stir in garlic, salt, and pepper. Cook until hash browns are crispy, 3 to 5 minutes. Spread Cheddar cheese evenly over the hash browns and let melt.