We like to eat these delicately flavored mussels with rice, but thin rice noodles are also good. A generous squeeze of lime at the end is essential.
Heat a large pot over medium-high heat. Add oil and heat until shimmering. Cook lemongrass and onion, stirring frequently, until onion begins to caramelize and turn golden, about 8 minutes.
Stir in chiles, coconut milk, wine, fish sauce, and herb stems. Bring mixture to a simmer.
Add mussels and cover pot. Cook until mussels open wide (discard any unopened ones), 4 to 5 minutes.
Transfer to a serving bowl. Sprinkle with herb leaves. Serve with lime and rice.
*Find arbol chiles in Mexican markets or your grocery store's Latino-foods aisle. If the mussel "beard" (stiff threads protruding from the shell) is still attached, pull it off.
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