Preheat your oven to 350°F (175°C).
Grease a baking dish with butter.
Peel and slice the potatoes thinly.
In a skillet, brown the ground meat with diced onions until cooked through.
Layer half of the sliced potatoes in the baking dish.
Spread half of the meat mixture over the potatoes.
Sprinkle half of the shredded cheese over the meat.
Repeat the layers with the remaining potatoes, meat, and cheese.
In a bowl, mix milk, sour cream, garlic powder, thyme, salt, and pepper.
Pour the mixture evenly over the casserole.
Cover with foil and bake for 40 minutes.
Remove the foil and bake for an additional 20 minutes, until golden brown.