Bring a large pot of water to the boil, add potatoes and simmer for 10 minutes. Drain the water from the potatoes keeping the potatoes in the pot. Add the butter, thyme and lemon zest to the potatoes. Place a lid on the pot and shake until the outside of the potatoes are roughed up and completely covered in butter.
Place on a flat baking dish and cook in a preheated 180°c oven 40 to 50 minutes. Remove from the oven and place in a serving dish and sprinkle with sea salt and sprigs of fresh thyme. Serve immediately with your favourite roast meat.