1. Combine the onion, garlic, herbs and arugula in the bowl of a food processor and pulse until very finely chopped.
2. Transfer the mixture to a large mixing bowl along with the turkey, salt and pepper. Use your hands to thoroughly combine all ingredients. Roll the mixture into golf ball-sized meatballs.
3. Place the marinara sauce in a large pot set over medium-low heat and let it warm.
4. Heat the olive oil in a large skillet over medium-high heat. Cook the meatballs in batches until they are browned all over, about 2-3 minutes per side. I usually need to do at least 2 batches so as not to overcrowd the pan.
5. Transfer the browned meatballs to the tomato sauce, which should be simmering. Partially cover the pot. Let the meatballs cook for 30 minutes, stirring occasionally to ensure that they cook evenly.
Serve with pasta, polenta or whatever else you like!
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