Melt butter in a large stock pot over medium heat. Add garlic and let sizzle for about 30 seconds. Season with red pepper flakes and lemon zest, stirring for about 45 seconds.
Quickly pour in wine into the pan and season with black pepper. Bring sauce to a boil, stir in mussels, and cover immediately. Shake pot and let boil for 1 minute.
Stir mussels, replace cover, and let boil for 2 more minutes. The shells will begin to open. Stir in parsley, cover pot, and cook until all shells are open, 1 to 3 minutes.
