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 fibrous patch of hairs, 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, 1 tsp 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.
* If you enjoy seafood with a lot of heat, taste the broth mixture. If you would like additional spice, you can add up to one more teaspoon of sriracha. But keep in mind that the additional heat will diminish the delicate sweetness of the mussels.