Pungent wrapped heart mustard (dai gai choy) is delicious alongside mussels in a traditional Thai sauce of green-curry paste and coconut milk.
Bring a medium pot of water to a boil. Add salt to boiling water, then add cabbage; cook until crisp-tender, about 2 minutes. With a slotted spoon, transfer greens to a colander to drain.
Bring stock to a simmer in a large saucepan over medium-high heat, and add mussels. Cover pan, and cook until mussels open (check pan frequently after 2 minutes). With a slotted spoon, transfer mussels as they open to four large soup bowls, dividing evenly. (Discard any mussels that remain unopened after 5 minutes.) Divide reserved greens among the soup bowls.
Pour broth from pan through a fine sieve into a bowl, and rinse pan. Return broth to pan, and stir in green-curry paste. Bring to a simmer over high heat. Add coconut milk, whisking until incorporated. Bring to a bare simmer.
Divide broth among bowls, pouring over mussels and greens to reheat. Serve with lime wedges. Garnish with garlic chives, if desired.
You can make this dish with bok choy in place of the wrapped heart mustard.