Preheat your oven to 375°F (190°C).

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

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. Stir until well combined.

In a greased 9x13 inch baking dish, spread half of the hashbrowns evenly across the bottom. Spoon the chicken mixture over the hashbrowns and then add the remaining hashbrowns on top, followed by the rest of the cheddar cheese and grated Parmesan.

Bake in the preheated oven for 30-35 minutes, watching for bubbling edges and a golden brown topping. If the top browns too quickly, cover loosely with aluminum foil.

Remove the casserole from the oven and let it rest for about 5 minutes. Before serving, sprinkle with chopped fresh chives or green onions.