Preheat the oven to 400°F and line two sheet pans with parchment paper.
In a medium skillet, heat the olive oil over medium heat until shimmering. Add the onion and garlic and cook, stirring occasionally, until the onion is translucent and softened, 5 to 7 minutes. Transfer to a large bowl and set aside for a few minutes to cool slightly.
Add the ground chicken, onion mixture, Parmesan, ricotta, breadcrumbs, salt, pepper, egg, and parsley to a large mixing bowl. Mix until just combined, being careful not to overmix.
Using a 3-tablespoon cookie scoop or your hands, form into meatballs that are about 2-inches in diameter. Place them on the prepared sheet pans with a little space between each.
Bake until the internal temperature registers 165ºF on an instant-read thermometer and the meatballs are lightly golden, about 25 minutes. If you like, broil each sheet pan for 2 to 3 minutes at the end to get the tops of the meatballs extra golden brown.
In a large skillet or pot, heat the marinara sauce over medium-low heat. Add the meatballs and sprinkle with freshly grated Parmesan. Enjoy!
Note: These freeze really well! Let the meatballs cool completely, then freeze them before adding to the sauce in a freezer-safe bag or airtight container. You can microwave them directly from frozen for an easy meal.