Heat 2 tbsp of the butter in a heavy pan over medium-high heat. Add almonds and pistachios; cook, stirring, until light brown, about 4 minutes. Remove and set aside.
Combine the garlic, ginger, coriander, cardamom, chili, peppercorns, cloves, cumin, mace and water in a blender and blend to form a smooth paste.
Arrange lamb in a large shallow dish. Rub the spice mixture over both sides of lamb. Cover and refrigerate 30 minutes.
Heat remaining 2 tbsp butter in a large skillet over medium-high heat. Add lamb and brown well on both sides. Add 1/4 cup of the water and bring to a boil. Reduce heat, cover, and simmer for 5 minutes. Add remaining water; sprinkle in saffron. Cover and cook until lamb is tender, about 10 minutes. Add salt to taste. Stir in peas and cook 5 minutes longer. Sauce should be very thick.
Transfer lamb to a heated platter. Stir sauce and pour over lamb. Top with fried nuts and cilantro.