Sweet potatoes could sub for the white potatoes here. And white pepper could substitute for the cayenne pepper making the dish nightshade-free. Another alternative, leave out the cayenne and let people add hot sauce individually at the table (what we did).
If you’ve got leftover lamb, but not the veggies, just cook fresh veggies in the seasoned broth for the appropriate time (about 20 minutes for the potatoes, a few minutes for the green veggies) before adding the lamb and yogurt.