Usually the mussels will be debearded when purchased, however, if the mussels still have their "beards" (byssal threads), wait until within an hour of cooking to remove the mussel beards by giving them a sharp pull toward the pointed tips of the mussels. Rinse the mussels under fresh, running water before cooking, then set aside. if any mussels are gaping open, the are getting weak, discard any that will not attempt to stay closed after squeezing their shell shut or if they have broken shells or an "off" odor.
Sauté scallions, tomatoes, garlic, and chili flakes in olive oil over medium heat until scallions begin to look clear. Add beer or wine, mussels, cilantro and juice from 1/2 lemon, cover and turn heat to high. Mussels should steam until they are all opened and the meats are no longer translucent, discard any mussels that will not open. Spoon mussels into serving bowls and ladle sauce over the top, then serve immediately. (Note: the longer the chili flakes are sautéed, the hotter they get, so put them into the mixture according to taste.) Accompany with bread and a cold ale.