A full bunch of vitamin-rich kale gives these turkey meatballs their vibrant green color. Rolled oats are used as a binder in place of breadcrumbs and add an extra bit of heart-healthy fiber.
Pulse the oats in a food processor until coarsely ground. Add the kale, 1 teaspoon of the minced garlic and ¼ cup of the chopped onion, and pulse until finely chopped and the texture is crumbly. Add the egg, 2 tablespoons of the Parmesan, milk, ¾ teaspoon salt and ¼ teaspoon pepper, and pulse until just combined. Transfer the mixture to a large mixing bowl, add the turkey and mix with your hands until well combined. Cover with plastic wrap, and refrigerate for 30 minutes.
Meanwhile, bring a large pot of water to a boil.
Heat the oil in a large, straight-sided skillet or Dutch oven over medium-high heat. Add the remaining onions and ¼ teaspoon salt, and cook, stirring occasionally, until softened and lightly browned in spots, 6 to 8 minutes. Add the remaining garlic, and cook until softened, stirring constantly, about 30 seconds. Add the tomatoes, 1 ½ cups water, 2 tablespoons of the remaining Parmesan, oregano, 1 teaspoon salt and the red pepper flakes. Stir to combine, and bring to a simmer. Cover, reduce the heat and simmer for 10 minutes.
Meanwhile, remove the turkey and kale mixture from the refrigerator. Shape level tablespoonfuls into round meatballs (about 36), and place on a rimmed baking sheet.
Remove the lid from the sauce. Use a metal spatula or 2 spoons to add the meatballs, one at a time, to the simmering sauce in a single layer. Cook, covered, until the meatballs are cooked through, about 15 minutes. Turn off the heat, and let the mixture sit, covered, for 5 minutes. Discard the oregano.
Meanwhile, cook the pasta in the boiling water according to package directions. Drain well.
To serve, divide the pasta among 6 plates or bowls. Top each serving with 7 meatballs and ½ cup tomato sauce. Sprinkle with the remaining Parmesan.