Preheat your oven to 375°F (190°C). Grease a medium baking dish with butter or oil.

If using raw potatoes, dice them into roughly 1/2-inch pieces for even cooking. If using pre-cooked or frozen diced potatoes, thaw and pat dry.

In a large bowl, combine the diced potatoes, sliced smoked sausage, and chopped onion. Season lightly with salt and pepper.

In a separate bowl, whisk the milk with the garlic powder. Stir in the shredded cheese, reserving a small handful to sprinkle on top.

Pour the milk-and-cheese mixture over the potato-sausage mix. Toss gently until everything is evenly coated.

Transfer the mixture to the prepared baking dish. Smooth into an even layer and sprinkle the reserved cheese on top.

Bake uncovered for 30–40 minutes, or until the potatoes are tender when pierced with a fork and the top is bubbling and beginning to brown. If the top browns too quickly, tent loosely with foil.

Let rest for 5–10 minutes before serving so the casserole sets and is easier to portion.