In a medium bowl, combine the curry powder and 1/2 teaspoon salt. Add the lamb and toss to coat. In a large saucepan, melt 2 tablespoons butter over medium-high heat. Add the onion, lamb and any remaining seasoning mixture to the pot and cook, stirring occasionally, until the lamb is browned, about 5 minutes. Stir in 1 1/2 cups water, scraping up any browned bits. Bring to a boil, cover, lower the heat to medium-low and cook until the lamb is tender and the sauce is thickened, about 45 minutes.
Meanwhile, in a medium saucepan, melt the remaining 1 tablespoon butter over medium-high heat. Stir in the rice and cook until lightly toasted, about 2 minutes. Add 2 cups water, bring to a boil, cover, lower the heat and simmer until tender, about 20 minutes.
In a large saucepan, combine the sweet potatoes with enough salted water to cover, then bring to a boil and cook until tender, 5 to 10 minutes; drain.
Add the sweet potatoes, coconut milk and papaya to the lamb mixture and cook until heated through. Stir in the cilantro and serve over the rice.