Add 1 1/2 pounds ground turkey, minced onion, minced garlic, 1 cup breadcrumbs or panko, 1 large egg, 1 teaspoon kosher salt, 1/2 teaspoon freshly ground black pepper, and 1/2 teaspoon ground allspice to a large bowl. Mix together with a wooden spoon or your hands until evenly mixed. Form the mixture into 1 - 1 ½ inch meatballs.
Bring 3 cups reduced-sodium chicken broth to a boil over medium-high to high heat in a saute pan or Dutch oven. Add the meatballs to the broth and reduce the temperature to medium. Simmer the meatballs in the broth until they are cooked through, 8-10 minutes.
Transfer the meatballs from the broth to a large, clean plate using a slotted spoon. Pour the broth into a large measuring cup and set aside. Wipe the pan clean or allow it to dry.
Curry Sauce
Cut off the bottom of the leek, and thinly slice it from the bottom. Use only the white and light green parts of the leek, discard any tough, dark green outer leaves, as well as the dark green tops. Place the sliced leeks in a salad spinner and immerse in cold water. Swish the leeks around to remove any dirt between the layers of leeks. If the leeks were particularly dirty, change the water once. Discard the water and spin the leeks until dry.
Melt 2 Tablespoons butter in the saute pan or Dutch oven over medium heat. Add the sliced leeks and 1/2 teaspoon kosher salt and cook until soft, about 6 minutes. Sprinkle 2 Tablespoons all-purpose flour and 1 Tablespoon curry powder over the leeks, and stir. Cook the flour until it starts to turn brown, and the curry powder is fragrant about 2 minutes.
Very gradually add the reserved chicken broth, stirring while you add it to prevent any lumps. Once all of the broth has been added, add the meatballs and simmer, uncovered for about 8 minutes until the sauce has thickened.
Stir in 1/2 cup whipping cream, and cook for about 2-3 more minutes. Remove the pan from heat and stir in the chopped apple and 2 Tablespoons minced fresh dill. Serve over cooked rice, and garnish with more dill if desired.