I'd cook a chopped onion and some garlic in a little oil, brown the lamb shank, add 4 cups water or broth and 3/4 cup lentils; bring to a boil, cover, turn to low, simmer for 30 minutes. At 30 minutes take the lid off and check the water level: add more if you want it soupier, otherwise keep simmering (turn up heat if necessary) for another 15 minutes. Pull out the shank and shred the meat, add to the lentils; add salt and pepper to taste. Cinnamon, cardamom, or cumin would be good in this if you like them. Also a chopped carrot (add for the last 15 minutes).