Bring a large pot of salted water to a boil. Add the pasta and cook until al dente. Reserve 1 cup of the pasta water and set aside. Drain the pasta and then return it to the same pot.
Over medium heat, add the butter and lemon zest to the pasta in the pot, add about ½ cup of the reserved pasta water, and season with sea salt and black pepper. Mix the pasta in the pot until the pasta is well coated with the mixture. If the pasta appears dry, add a little more of the reserved pasta water to the pot until you get the desired consistency.
Turn the heat off and add the juice of half the lemon, stir well to combine.
Divide the pasta into two serving bowls.
Top with grated cheese, chopped parsley, and red pepper flakes (optional).
Serve immediately.