1. Place the 2 peeled whole garlic cloves, 2 teaspoons of the grated ginger, and the water in a blender. Blend until smooth, about 25 seconds; leave the mixture in the blender jar and set aside. (You will be pureeing the soup right in the blender with the garlic and ginger.)
2. Heat a large stockpot or Dutch oven over medium-high heat until very hot. Add 1 tablespoon of the olive oil, swirl to coat the pan bottom, and add half of the lamb pieces. Cook until the lamb is well browned, about 2 to 3 minutes. Transfer the browned lamb to a medium bowl with a slotted spoon. Add the remaining tablespoon of oil to the pot, swirl to coat the pan bottom, add the remaining lamb, and cook until the lamb is well browned on all sides. Return all the lamb to the pot and add the chicken broth, scraping up the browned bits from the pan bottom with a wooden spoon. Bring to a simmer, cover, reduce the heat to medium-low, and simmer until the lamb is tender, about 20 minutes. With a slotted spoon, transfer the lamb to a medium bowl, cover, and reserve. Pour the broth into another bowl and reserve.
3. Heat the butter in a large stockpot or Dutch oven over medium heat until foaming. Add the onions and tomato paste and cook, stirring frequently, until the onions are softened and beginning to brown, about 3 minutes. Stir in the coconut and cook until fragrant, about 1 minute. Add the minced garlic, remaining 2 1/2 teaspoons ginger, curry powder, cumin, cayenne, and flour; stir well until evenly combined, about 1 minute. Whisking constantly and vigorously, gradually add the reserved broth used to cook the lamb
4. Add the carrots, celery, and whole banana to the stock. Increase the heat to medium-high and bring to a boil. Cover, reduce the heat to low, and simmer until the vegetables are tender, about 20 minutes.
5. Puree the soup in batches in the blender with the garlic and ginger until very smooth. Wash and dry the pot. Return the pureed soup to the clean pot and season to taste with salt and pepper. Add the lamb pieces and warm the soup over medium heat until the lamb is hot, about 5 minutes. Ladle the soup into individual bowls, spoon a dollop of the yogurt over each bowl, sprinkle with the cilantro, and serve immediately.