Allow hash browns to thaw just enough so there are no lumps or big frozen chunks, they can still be partially frozen.
Place butter in casserole dish and melt in the microwave.
Add hash browns, onion, cheese, and can of soup to the dish in that order. Fold to combine, making sure there are not lumps of potato or cheese, smooth out in dish.
Place in oven and bake at 350 degrees for 1 hour.
Notes
Note: This can be made the day before, keep covered in refrigerator until ready to bake.