Preheat oven to 200°C (400°F). Grease a 9x13-inch casserole dish.
Arrange potatoes in a single layer in the dish. Scatter onion and bacon over potatoes, then dot with diced butter. Cover with foil.
Bake for 45 minutes. Remove foil, then switch to broiler setting. Stir potatoes every minute or so for even browning until crisp, about 5 minutes.
Serve immediately.
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