In a pot of boiling salted water, cook the tagliatelle until al dente. Drain the pasta, reserving 1/2 cup of the cooking water. Return the pasta to the pot. Add 1/2 cup of Parmesan and the butter, thyme, lemon zest and the reserved cooking water. Cook over moderate heat, tossing just until creamy. Season with salt and pepper and transfer to a bowl. Sprinkle with the remaining Parmesan and serve.