Preheat the oven to 200°C/fan180°C/gas 6.
Boil the potatoes in a large pan of water. Allow to simmer for 10 minutes. Drain away the water.
Put the oil, butter, whole garlic cloves and sage leaves in a roasting tin in the oven. Heat for a few minutes. Add the potatoes and gently squash each one with a fork. Sprinkle with salt and black pepper and roast for 40 minutes turning them half way through cooking.
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