Rinse the mussels well in cold water, discarding any that won’t close when handled. If the mussels have not been de-bearded, remove the patch of seaweed, or “beard,” sticking out from between the two shells by pulling with your fingers. If it's stubborn, cut it with a knife.
In a small bowl, whisk together the mirin, chicken broth, sriracha and fish sauce.
In a nonstick sauté pan or wok, heat the oil over medium-high heat.
Sauté the garlic in the oil until edges brown, about 30 seconds.
Add the liquid mixture. Bring to a boil and boil for 1 minute.
Add the mussels, reducing heat slightly, cover and cook until all the mussels are open, about 4-5 minutes. (They may open sooner.) Remove from heat. Drizzle with lemon juice and top with scallions and/or cilantro. Or, if you prefer, just eat them naked—the way I like to do it.
Divide mussels between 2 bowls and serve with a crusty baguette to sop up the juice.