Heat oil in a heavy pot over medium-high heat. Add the lamb cubes and bones and brown on all sides, about 3 minutes. Sprinkle with salt and pepper. Add onion and garlic and continue cooking about 2 minutes, stirring frequently.
Stir in the lentils, 2 cups of the chicken broth, tomatoes, carrots and the thyme, sage and basil. Bring to a boil, reduce the heat to low and simmer, covered, until the lentils are tender but not quite done, stirring occasionally. Depending on the type and freshness of the lentils, this will take from 15 to 30 minutes. If the stew seems dry, add more broth (up to one cup additional). Discard the lamb bones.
Add the spinach and cook 5 minutes more. Stir in the lemon zest and juice. Garnish individual servings with some of the crumbled cheese.
