Preheat the oven to 375°F. Heat the olive oil over medium heat and cook the fully cooked kielbasa and onion for 6–8 minutes until the onion softens and sausage edges brown.
Combine the 1/2-inch diced potatoes, sausage, onion, milk, garlic powder, paprika, salt, and pepper. Spread in a lightly greased baking dish in a shallow layer and top with all the shredded cheese.
Bake
Cover tightly with foil and bake at 375°F for 30 minutes. Uncover and bake about 15 minutes more until the center potatoes are fork-tender and the casserole reaches 165°F, covering again if more cooking is needed.
Let stand for 5 minutes. Divide into 6 portions with some of the pan juices.
Notes
Refrigerate promptly in shallow covered containers at 40°F or below; use within 3 days of the oldest cooked ingredient.