Add the stock, fish sauce, garlic, chile, lemongrass and shallot to a 6- to 8-quart slow cooker, cover and cook on high for at least 2 hours. Add the mussels and coconut milk and continue to cook on high, covered, until the mussels open, another 15 to 20 minutes. Discard any unopened mussels. Stir in the cilantro and lime zest and juice. Add more fish sauce if desired.
Ladle into bowls and serve with Crusty Gochujang Garlic Bread.
Crusty Gochujang Garlic Bread:
Preheat the broiler. Mix together the butter, gochujang and garlic in a small bowl. Smear the butter mixture on the baguette halves and place on a baking sheet. Broil until golden and crispy, 3 to 5 minutes.