Rinse the mussels under cold running water. Pull off any stringy beards by tugging them toward the hinge end of the shell. Discard any mussels that are cracked or that do not close when you tap them on the counter. Set aside in a colander.

Melt 2 tablespoons of the butter in a large heavy-bottomed pot over medium heat. Add the minced garlic and red pepper flakes if using. Cook, stirring constantly, for about 1 minute until the garlic is fragrant but not browned. Browned garlic turns bitter, so watch it closely.

Pour in the white wine and raise the heat to medium-high. Let the wine come to a simmer, which takes about 2 minutes. Stir in the lemon juice and the remaining 1 tablespoon of butter. Taste the broth and add salt as needed.

Add all the mussels to the pot in one go. Give them a quick stir to coat them in the broth, then put the lid on tight. Cook on medium-high for 6 to 8 minutes. Resist the urge to lift the lid. You want that steam to build up and pop those shells open.

After 6 minutes, lift the lid and check. Most mussels should be wide open. Give the pot a gentle shake and cover for another minute or two if a few are still closed. Any mussel that has not opened after 8 minutes total should be discarded, not eaten.

Scatter the chopped parsley over the top and give everything one more stir. Spoon the mussels and broth into wide bowls and serve immediately with lemon wedges and plenty of sliced baguette on the side for soaking up the broth.