Heat the olive oil in a skillet. Add the onion and cook until translucent and just starting to brown, about 7 minutes. Add the spinach, garlic and some salt and cook until the spinach is wilted. Add the fennel seeds and cook for another minute, then transfer to a large bowl. Add the turkey, breadcrumbs, parsley, cayenne, onion powder, oregano, paprika, thyme, egg, 1 teaspoon salt and some black pepper and mix. Roll into golf ball-sized balls.
Preheat the oven to 425 degrees F.
Heat a large skillet over medium-high heat and coat with about 1/2 inch of oil. Brown the meatballs on all sides in batches, being careful not to overcrowd the skillet, 6 to 10 minutes total. Finish the meatballs in the oven until cooked through, 7 to 10 minutes. Serve with Yogurt Sauce and Chimichurri.
Add the yogurt, garlic and some salt and pepper to a small bowl. Mix to combine.
To the carafe of a blender or food processor, add the cilantro, parsley, olive oil, garlic, lemon juice and some salt, black pepper and crushed red pepper. Pulse until combined but still slightly chunky, adding more olive oil as needed to loosen. Taste and adjust the seasoning.