Grease the slow cooker: Brush the inside of the slow cooker insert generously with butter or spray with non-stick cooking spray.

Make sauce: Whisk together cream of mushroom soup, chicken broth, sour cream, garlic powder, onion powder, smoked paprika, dried thyme, black pepper, and salt. Add 1 cup of shredded cheddar and minced garlic. Stir to combine.

First potato layer: Place half the sliced potatoes in the bottom of the slow cooker in an even overlapping layer. Season lightly with a pinch of salt and pepper.

Kielbasa and onion layer: Scatter half of the sliced kielbasa evenly over the potato layer. Add half of the sliced onion rings over the kielbasa.

Add half the sauce: Pour half of the prepared cheese sauce evenly over the kielbasa and onion layer. Use a spoon to spread it to the edges of the slow cooker insert.

Repeat layers: Add the remaining potato slices in a second even layer. Add the remaining kielbasa and onion rings. Pour the remaining cheese sauce over the top, spreading it evenly to cover everything.

Cook: Cover and cook on LOW for 6 to 7 hours until potatoes are completely tender and sauce is thick and bubbly.

Add cheese topping: Sprinkle the remaining 1 cup of shredded cheddar cheese evenly over the entire surface of the dish. Replace the lid and cook on HIGH for 15 to 20 minutes until the cheese is completely melted, bubbly, and starting to develop golden patches.

Garnish and serve: Spoon into bowls or onto plates. Garnish with sliced green onions, fresh parsley, and a dollop of sour cream. Serve immediately while the cheese is still bubbling.