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Combine all of the ingredients, except the sour cream, in the blender jar or food processor bowl.
Cover and process until the potatoes are finely ground, stopping to scrap down as necessary.
Pour into a buttered, 20cm square nonstick baking dish. Bake in a preheated, 170°C oven, 1 hour to 75 minutes, or until golden brown and set.
Cut into squares and serve with a dollop of sour cream.