Preheat your oven to 400°F (200°C). This temperature is ideal for baking meatballs, allowing them to cook evenly and become golden brown.

In a large bowl, combine the ground turkey, finely chopped parsley, minced onion, ricotta cheese, and half of the grated Parmesan. Adding ricotta gives the meatballs a creamy texture, while the onion infuses flavor.

Now, add the breadcrumbs, beaten egg, Dijon mustard, lemon zest, sage, red pepper flakes (if using), salt, and black pepper. These ingredients help bind the mixture while providing flavor. Mix everything gently using your hands. Be careful not to overmix as this can lead to tougher meatballs.

With clean hands, shape the mixture into meatballs, about 1 to 1.5 inches in diameter. Place them on a parchment-lined baking sheet for easy cleanup. You can choose how large or small you want your meatballs, adjusting the cooking time accordingly.

Sprinkle the remaining Parmesan cheese over the meatballs. Bake them in the preheated oven for around 20 to 25 minutes. Check for doneness; they should be cooked through and golden on the outside. An instant-read thermometer should register 165°F (74°C) in the center of a meatball.