Prep the spinach: Heat a small pan with a teaspoon of oil. Add the fresh spinach and cook until it wilts. Squeeze out extra water and chop finely. Let it cool.

Mix the base: In a large bowl, add the ground meat, cooled chopped spinach, minced garlic, finely chopped onion (if using), egg, breadcrumbs, grated Parmesan, Italian seasoning, red pepper flakes, salt, and pepper. Add chopped parsley last.

Combine gently: Use clean hands to mix the ingredients. Do not overwork the meat. Stop when all parts look even.

Form the meatballs: Take a small handful and flatten slightly. Place a chilled mozzarella cube in the center. Fold meat around the cube and roll into a round ball, aiming for about 1.5 inches in diameter.

Choose your cooking method: For pan-searing, heat a nonstick skillet over medium heat with a bit of oil. Add meatballs and cook 4–5 minutes per side, turning gently, until browned and cooked through. For oven method, place meatballs on a parchment-lined baking sheet and bake in a preheated 400°F (200°C) oven for 15–18 minutes or until cooked through and browned.

Check doneness: Use a meat thermometer if you have one. The centers should reach 160°F for beef or 165°F for turkey. Cut one open to ensure the cheese is melted and the meat is clear of pink.

Rest before serving: Let meatballs rest 3–5 minutes. This helps them set and keeps the cheese soft but not runny.