Preheat your oven to 400°F (200°C). Line a baking sheet with parchment paper or lightly oil an ovenproof skillet.

Heat a tablespoon of olive oil in a skillet over medium heat. Add the finely chopped onion (if using) and sauté until translucent, about 3–4 minutes.

Add the minced garlic and cook another 30–45 seconds until fragrant.

Add the chopped fresh spinach and cook until wilted and any excess moisture has evaporated, about 2–3 minutes. Remove from heat and let cool for a few minutes.

In a large mixing bowl, place the ground beef or turkey. Add the cooled spinach mixture, breadcrumbs, grated Parmesan, Italian seasoning, red pepper flakes, chopped parsley, beaten egg, and a generous pinch of salt and pepper.

Use your hands or a spatula to gently combine until just combined. Don’t overmix — overworked meat will lead to dense meatballs.

Ensure your mozzarella cubes are chilled to make them easier to encase.

Portion the mixture into even balls using a small cookie scoop or a tablespoon. Aim for 1 1/4- to 1 1/2-inch meatballs.

Flatten each portion slightly in your palm, place a chilled mozzarella cube in the center, fold the meat around the cheese, and roll gently to seal.

Place the assembled meatballs on the prepared baking sheet, spacing them about an inch apart.

For a golden exterior, lightly brush the tops with olive oil.

Bake in the preheated oven for 15–20 minutes, until browned on the outside and cooked through.

Let the meatballs rest for 5 minutes after baking; this helps the juices redistribute.

Serve warm with your favorite sauce or sides and garnish with extra parsley and grated Parmesan.