Heat oven to 220C/fan 200C/gas 7. Peel the potatoes and place them in
a pan of cold, salted water. Bring to the boil, simmer for 3 mins and drain. When cool enough to handle, slice a few very neatly. Coat the bottom of
a round, ovenproof, non-stick pan with butter, then place a sprig of thyme in the middle. Cover with a slice of potato in the middle, then arrange the rest of the slices over the base of the pan, overlapping the slices in concentric circles. Slice the rest of the potatoes and tip into a bowl, then pour over the rest of the butter, the nutmeg and the thyme leaves and season with salt and pepper. Toss everything together, then tip the seasoned potatoes over the arranged slices and press down gently to flatten. The potato cake can be prepared up to a day ahead and kept covered in the fridge.
Cook the cake in the oven below the beef. Every now and then, remove it, press it down with a fish slice and drizzle the surface with a little olive oil. Cook for 1hr-1hr 15mins, until the sides look brown and crispy. Carefully invert the cake onto a serving dish or board, scatter with sea salt and serve cut into wedges.