Heat a large skillet over medium-high heat and stir in the ground beef. Cook and stir until the beef is crumbly, evenly browned, and no longer pink. Drain and discard any excess grease. Stir in the spinach and cook until wilted, about 1 to 2 minutes. Remove from heat and allow to cool for 10 minutes.

Transfer the beef mixture to a bowl. Add the Parmesan, parsley, bread crumbs, olive oil, egg, garlic salt, and pepper and mix well. Run the filling through a grinder until smooth (or puree in a food processor until smooth).

The filling can be kept in the refrigerator for up to four days or in the freezer for up to three months.