Preheat your oven to 375°F (190°C). Grease a 9x13 inch casserole dish.
Boil diced potatoes in salted water until tender, about 15 minutes. Drain and set aside.
Meanwhile, season the chicken breasts with garlic powder, onion powder, salt, and black pepper.
Cook seasoned chicken breasts in a skillet over medium heat until no longer pink inside, about 7-8 minutes per side. Let rest, then shred.
In a large bowl, combine shredded chicken, cooked potatoes, sour cream, milk, 1 cup of shredded cheddar, and melted butter. Mix gently until well combined.
Transfer the mixture to the casserole dish. Sprinkle the remaining 0.5 cup shredded cheddar cheese on top.
Bake uncovered in the preheated oven for 30 minutes or until cheese is melted and bubbly.
Remove from oven and garnish with chopped green onions before serving.
Notes
This casserole can be made ahead and refrigerated overnight before baking. Leftovers keep well for up to 3 days.