1) Preheat oven to 180°C / gas mark 4. Lightly butter a casserole dish.
2) In a small frying-pan, melt 60g butter over medium heat. Add onion, and cook three to four minutes, or until soft. Pour mixture into a large bowl. Add the sour cream, cream of celery soup, cheese, garlic powder, hash browns, and salt and pepper. Combine until well blended.
3) Pour mixture into prepared baking dish. Top evenly with crushed potato crisps and bake for 45 minutes, or until hot and bubbly.