Combine the lamb, salt, pepper (about 1/2 teaspoon), ginger, cardamom, fennel, and cayenne in a food processor. Process, stopping the machine and scraping down the sides if necessary, until the meat is very smooth. Add about 1/4 cup of the yogurt and process until even smoother. Bring a large pot of water to a boil and add salt.
Shape the meat into large balls, 1 1/2 inches or so in diameter, and add to the boiling water; adjust the heat so the mixture simmers and cook until the meatballs swell, about 15 minutes.
Put the butter in a large skillet or flameproof casserole with a lid over medium heat. A minute later, add the curry powder and stir for 30 seconds. Add the milk and remaining 1 1/4 cups yogurt, stir until smooth, and bring to a gentle simmer. Add the meatballs, then cover and adjust the heat so the mixture simmers steadily. Cook for another 15 minutes, then serve, garnished with the mint.
