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Heat oven to 190C/170C fan/gas 5. Dry-fry the bacon in a pan for 5 mins until crisp, then chop into small pieces. Boil the whole potatoes for 5 mins, drain, then place in a bowl of chilled water.
When cool enough to handle, pat the potatoes dry and roughly grate into a large bowl. Toss with the oil as you go, to stop them from sticking. Roughly grate the onion and squeeze out any excess juice, then stir into the potatoes along with the bacon.
Place a baking sheet in the oven for 5 mins to warm through. Liberally grease a 23cm loose-bottomed cake tin with butter. Scatter the potato over the tin, trying not to pack it down, then dot all over the top with butter. Place on the hot baking sheet and bake in the oven for 1 hr 20 mins until the potatoes are cooked through and crisp on top.