Put the potatoes in a pan of lightly salted water then bring to the boil. Simmer for 15 minutes, until tender but not totally cooked through. Drain and let cool (this can be done in advance)
Cut the potatoes into large chunks and put them into an oven-proof dish. Lightly squash them with a potato masher, then crumble over the butter. Season with salt and pepper and bake in a preheated oven for 25-30 minutes, until lightly browned.
Now crush them with the masher or a fork again then stir through the garlic and thyme leaves. Drizzle with olive oil, season with salt and pepper, then bake for a further 20-25 minutes until golden and crispy.
Serve warm.