Preheat the oven to 375°F.
Lightly grease a 9x13-inch baking dish.
Add the sliced potatoes to a large pot of salted water and boil for 8 to 10 minutes, just until slightly tender.
Drain well.
In a large bowl, mix melted butter, olive oil, ranch seasoning, garlic powder, paprika, black pepper, and salt.
Add the potatoes, smoked sausage, onion, and garlic to the bowl.
Toss until everything is well coated.
In a small bowl, stir together sour cream, heavy cream or milk, 1 cup cheddar cheese, and mozzarella cheese.
Spread half of the potato and sausage mixture into the prepared baking dish.
Spoon half of the creamy cheese mixture over the top.
Add the remaining potatoes and sausage, then spread the rest of the creamy mixture over everything.
Sprinkle the remaining 1 cup cheddar cheese on top.
Bake uncovered for 30 to 35 minutes, until the potatoes are tender, the cheese is melted, and the edges are golden.
Let rest for 5 minutes, then sprinkle with parsley.
Serve warm with green beans, salad, or dinner rolls.
Store leftovers covered in the refrigerator for up to 3 days.
