Return the lamb to the skillet, add the potato and cook until sizzling. Add the curry powder, cumin, coriander and cayenne and cook over low heat, stirring constantly, just until slightly darkened, about 4 minutes. Add the broth and bring to a simmer, stirring, to scrape up any browned bits from the bottom. Cover the skillet and cook over low heat until the meat and potato are tender, about 30 minutes. Remove the lid and cook until the liquid is absorbed. Stir in the peas and season with salt. Transfer to a bowl and let cool completely.