Combine the turkey, onion, egg, breadcrumbs, cheese, garlic, salt, pepper, and parsley in a large bowl. Mix with your hands until all of the ingredients are evenly distributed.
Begin heating a large pot of salted water for the cooking the greens while you shape and cook the meatballs.
Roll the meat mixture into 1 1/4-inch meatballs and place on a tray or in a pan. You should have between 26 and 30 meatballs.
Heat the olive oil over medium-high heat in a large skillet or sauté pan. Place the meatballs in the hot skillet and brown for 3 to 4 minutes on each side. Use tongs to gently rotate the meatballs so they brown evenly. (If your pan isn't large enough to brown all of the meatballs without them touching, cook them in batches and place them back on the cookie sheet once they've been browned. Then combine them back in the pan before simmering with the tomato sauce.)
Reduce the heat to medium, add the tomato sauce, and cover the pan. Simmer the meatballs and sauce for another 10 minutes, or until meatballs are cooked through.
While the meatballs are simmering, boil the greens in the salted water for 8 to 10 minutes, until tender. Drain. (You can also sauté or stir-fry the greens in a hot skillet with oil if preferred.)
To serve, put greens on the plate and top with meatballs and sauce.