Preheat the oven to Gas Mark 6, 200°C, fan 180°C. Bring the potatoes to the boil in a pan of salted water. Simmer for 10 minutes until just tender, then drain.
Add the oil and butter to a roasting tin and heat in the oven until the butter has melted. Add the potatoes and toss in the butter to coat, then gently squash each potato with a fork. Sprinkle with sea salt, black pepper, fennel seeds and rosemary and roast for 30 minutes.
Turn the potatoes over using a spatula or spoon, then add the garlic and roast for a further 20-30 minutes, until the potatoes are golden and crispy, and the garlic has softened.
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