Preheat the oven to 350°F. Line a baking sheet with parchment paper and set aside.
Gather the ingredients. Chop the spinach, parsley, red onion, and garlic. Measure out the breadcrumbs, cheeses, salt, and pepper. Beat the egg.
Add the ground chicken to a large bowl. Stir in the rest of the ingredients except the egg until everything is incorporated into the chicken. Add the egg and stir so the mixture comes together.
Scoop out some batter with your hands and roll it into a ball about 1.5-inches in diameter. Repeat with the rest of the batter until you get 15 balls of about the same size.
Place the balls in a single layer on the lined baking sheet. Bake on the center rack of the oven for 25 to 30 minutes, flipping halfway through, until lightly browned and fully cooked through. If you have a meat thermometor, you can check the meatballs for doneness. They should have an internal temperature of at least 165°F.
Enjoy the meatballs on their own with a dipping sauce, over pasta tossed with olive oil and parmesan cheese, in a meatball sub, or as part of a grain bowl or salad.
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Notes
If you want the meatballs to be a little crispier, consider pan frying them for a few minutes before baking. Warm some olive oil in a skillet, add the meatballs, and cook for a few minutes on each side before placing on the baking sheet. Finish cooking in the oven.
Store leftovers in an airtight container in the fridge for up to 5 days. Reheat in a skillet, covered with a lid, with a little olive oil. Cook for a few minutes on each side until warmed through.
To freeze, cook the meatballs completely, let cool, then transfer to freezer bags or containers. Arrange in a single layer(s), separated by parchment paper if needed. Freeze for up to 3 months. Thaw in the fridge before reheating or reheat from frozen in sauce of choice in a saucepan with lid until warmed through.