Assemble the butter, cream cheese, bacon, onion, seasoning salt, pepper and 1/2 cup shredded cheddar cheese. Mix until combined.
Add potatoes. These can be leftover mashed potato, potato cubes, or shredded potato. Mix well. Taste mixture. Add additional salt and pepper if needed.
Spray an 8x8 or 9x9 pan with non-stick spray. Add potato mixture to the pan.
Top with remaining 2 cups shredded cheddar cheese.
Bake for 45 minutes. Note: If using uncooked potatoes, you should cover the dish with foil for 30 minutes and then remove the foil and cook for an additional 30 minutes.