In a large (8-12 quart) heavy-bottomed stockpot or dutch oven, add the oil and three popcorn kernels, heating over high heat. Cover the pan. These are the "test kernels" and once they pop, it signals that the oil is officially hot enough to pop all the kernels at the same time. Meanwhile, measure out the remaining 1/2 cup of popcorn kernels and begin preparing your butter by melting it either in a microwave-safe bowl or on the stovetop.
Once the three test kernels have popped, immediately add the 1/2 cup of popcorn kernels to the pan. Cover and remove from heat, gently swirling from side to side for 20 seconds. Return the pan to the heat, and the popcorn kernels will almost immediately beginning popping.
Once the kernels are about halfway done popping, slightly crack the lid to allow excess steam to escape. Do this when the stockpot is approximately halfway full with popcorn, or about 30 seconds to 1 minute after they've popped – this does not have to be exact.
Once the popping has slowed or just stopped, immediately pour the popcorn into your serving bowl.
Add the lemon zest and red pepper flakes to the melted butter. Pour evenly across the bowl of popcorn, tossing to combine.
Add salt as desired, as well as freshly ground black pepper, if using. This is the perfect time to taste test to be sure the salt level is where you'd like. Sprinkle on the chopped parsley, again tossing to coat the popcorn with the parsley leaves. Enjoy immediately!