How to Make It
In a food processor, pulse the ginger with the garlic, chiles and 3/4 cup of the chopped cilantro until a finely chopped paste forms, about 2 minutes.
In a large saucepan, melt the ghee over moderately high heat. Add the cumin seeds and cook until fragrant and beginning to pop, about 1 minute. Add the onions and season with generous pinches of salt and pepper. Cook until softened and beginning to brown, about 10 minutes. Stir in the cilantro paste and cook until slightly softened, about 1 minute. Add the sweet potatoes, coriander, turmeric, kidney beans, the tomatoes and their juices and 2 cups of water. Bring to a boil, then simmer over moderate heat until the sweet potatoes have softened and the liquid has thickened, about 30 minutes. Stir in the remaining 3/4 cup chopped cilantro, the garam masala and lemon juice. Season with salt and pepper.
Ladle the curry into bowls. Serve with a dollop of yogurt and garnish with cilantro sprigs.