Summarize:

When the mussels are coated with the shallot mixture, add ½ cup (118 ml) of dry white wine, the juice from 1 lemon, 1 cup (237 ml) of chicken broth, and a pinch of red pepper flakes to the pot. Stir well to ensure that the flavors are fully blended. You can use any steaming liquid that you like for the mussels. You could use all white wine or chicken broth or substitute beer if you prefer. Even plain water will work, though the mussels won’t have as strong a flavor when they’re finished steaming. Once you’ve added the liquids to the pot, cover it. Allow the mussels to steam over medium-high heat for about 5 minutes or until they begin to open. When the mussels have opened, mix in ½ cup (15 g) of roughly chopped parsley and 2 tablespoons (28 g) of unsalted butter. Cover the pot again, and allow the mussels to steam for another minute so they soften. Once the mussels are finished steaming, use a slotted spoon to transfer them to serving platter. You can spoon some of the steaming liquid over them if you like, but be sure to serve them while they’re still hot.  If you prefer, you can serve the mussels directly from the pot that you steamed them in. If any of the mussels haven’t opened during steaming, discard them.
Pour the wine, lemon juice, chicken broth, and red pepper flakes over the mussels. Cover the pot and steam the mussels until they open. Add the parsley and butter and steam for another minute. Serve the mussels while still warm.