Prep the spinach. If using fresh, sauté it in a little olive oil with a pinch of salt until wilted, 1–2 minutes. Cool, then squeeze out excess liquid and chop finely. If using frozen, thaw completely and squeeze it dry with paper towels.

Make the panade. In a large bowl, combine breadcrumbs with the grated onion and any juices from grating. Stir in the egg, garlic, Parmesan, parsley or Italian seasoning, 1 teaspoon salt, and 1/2 teaspoon black pepper. Add a pinch of red pepper flakes if you like heat.

Add the turkey and spinach. Gently mix in the ground turkey and chopped spinach with a fork or clean hands. Do not overmix; stop as soon as it looks evenly distributed.

Shape the meatballs. Lightly oil your hands and form 18–22 golf ball–sized meatballs (about 1.5 tablespoons each). Place on a plate or sheet pan.

Sear for flavor. Heat 1–2 tablespoons olive oil in a large skillet over medium-high. Add meatballs in a single layer, without crowding. Sear 2–3 minutes per side until browned. Work in batches, adding oil as needed.

Simmer in sauce. Pour marinara into the skillet (or transfer meatballs to a pot with sauce). Reduce heat to low and simmer gently for 10–12 minutes, covered, until internal temperature reaches 165°F and meatballs are cooked through.

Taste and finish. Adjust salt and pepper in the sauce. Sprinkle with extra Parmesan and parsley if you like. Serve over pasta, zucchini noodles, polenta, or as a meatball sub.