In a large mixing bowl, combine the sliced kielbasa, diced potatoes, chopped onion, shredded cheddar cheese, cream of mushroom soup, milk, salt, and black pepper. Mix well.
Transfer the mixture to a baking dish and spread it out evenly.
Bake in the preheated oven for 45 minutes, or until the potatoes are tender and the top is golden brown.
Remove from the oven and let it cool for a few minutes before serving.
Notes:
This casserole is great for meal prep and can be stored in the fridge for up to 3 days.