Heat butter and oil in a large Dutch oven over medium heat. Brown lamb in batches; transfer to a plate. Discard all but 2 tablespoons fat.
Add onion to Dutch oven; cook until soft, about 5 minutes. Stir in garlic, ginger and curry powder. Cook, stirring, 1 minute more.
Return lamb and accumulated juices to Dutch oven. Pour in broth. Bring to a boil over high heat; reduce heat to low and simmer, covered, until lamb is very tender, about 1 1/2 hours. Add salt and freshly ground black pepper to taste.
Remove from heat. Stir in yogurt and cilantro. Before serving, garnish with pumpkin seeds.