Set your oven to 350°F (175°C) and grease a 9×13-inch baking dish.
Cook the beef: Brown the ground beef in a skillet over medium heat. Drain excess fat, then season with garlic powder, onion powder, salt, and pepper.
Make the cheese sauce: Whisk 1 ½ cups of milk with flour in a saucepan until smooth. Cook over medium heat, stirring constantly, until thickened. Remove from heat and stir in cheddar cheese until melted.
Assemble the casserole: Layer half of the sliced potatoes, ground beef, and onions in the baking dish. Pour half of the cheese sauce over the layers. Repeat with the remaining ingredients.
Add the final touch: Pour the remaining ½ cup of milk over the top.
Bake: Cover with foil and bake for 45 minutes. Uncover and bake for another 15-20 minutes until the potatoes are tender and the top is golden brown.
Serve: Let it rest for 5-10 minutes. Garnish with parsley, sour cream, or green onions.