Preheat your oven to 375°F (190°C).

Gather and prepare your ingredients: dice the onion, mince the garlic, and shred the cooked chicken.

Pat the frozen hashbrowns dry with paper towels to remove excess moisture.

In a large mixing bowl, combine the shredded chicken, sour cream, cream of chicken soup, and milk. Then add melted butter, diced onion, minced garlic, smoked paprika, salt, pepper, and half of the cheddar cheese.

In a greased baking dish, spread half of the hashbrowns evenly across the bottom. Spoon the chicken mixture over the hashbrowns, followed by the remaining hashbrowns, and top with the rest of the cheddar cheese and grated Parmesan.

Bake in the preheated oven for 30-35 minutes until bubbly and golden brown. Cover loosely with foil if the top browns too quickly.

Let the casserole rest for about 5 minutes before serving. Garnish with chopped chives or green onions.