Heat oven to 350°F. Spray baking dish with cooking spray.
Mix condensed soup, 1/2 cup sour cream, the garlic powder, salt, pepper and 1/2 cup of the cheese until mixed well. Stir in chicken (if using) and 1/2 cup of the green onions.
Place half of the frozen tater tots in single layer on bottom of your baking dish. Spread mixture on top of potatoes. Top with remaining potatoes.
Bake for 45 minutes. Top casserole with remaining 1/2 cup cheese. Bake 10 minutes longer, until cheese is melted. Top with remaining sour cream and top with remaining 1/4 cup sliced green onions.