Melt the butter in a medium-sized cooking pot.
Chop the leek and bok choi into small pieces and add to the butter. Cook over medium heat until the vegetables are tender.
Add the chicken and heat until warm.
Add 3 cups of broth and a pinch or two of salt and cook until bubbling.
Cover, and let simmer for at least 10-20 minutes for the flavors to blend.
Add the cooked lentils and return to a boil.
Add more salt and broth if desired.
Serve hot.