Bring a large pot of water to boil. Place the lemon zest, parsley and garlic on a cutting board. Chop until finely chopped and well combined, then set aside. Cook the Brussels sprouts in the boiling water until just tender, 5-6 minutes depending on size. Drain well and place in a serving dish. Drizzle with butter, season with salt and pepper, sprinkle over the gremolata and serve.