Prep Ingredients: Preheat your oven to 375°F (190°C) if baking. Chop spinach finely and mince garlic.
Sauté Spinach and Garlic:
Heat 2 tablespoons of olive oil in a skillet over medium heat.
Add minced garlic, sautéing until fragrant (about 1 minute).
Add spinach, stirring until wilted and dry (3-4 minutes).
Season with salt and pepper, then remove from heat to cool.
Mix Meatballs:
In a large bowl, combine ground beef, cooled spinach-garlic mixture, breadcrumbs, beaten eggs, milk, grated Parmesan, and a pinch of salt and pepper.
Mix until just combined—overmixing can make the meatballs tough.
Stuff with Cheese:
Form the mixture into balls (about the size of a golf ball).
Press a cube of mozzarella into the center of each ball, then reseal and roll to hide the cheese completely.
Cook:
For baking: Place meatballs on a greased baking sheet and bake for 20-25 minutes, until browned and cooked through.
For pan-frying: Heat a thin layer of oil in a skillet over medium-high heat. Brown the meatballs on all sides, then reduce heat and cover until cooked through (about 10 minutes).