Preheat oven to 400 degrees. Line a large baking sheet with aluminum foil or parchment.
To a large bowl, add the ground turkey, eggs, breadcrumbs, oregano, basil, cumin, mustard, salt, garlic powder, pepper, and cayenne, if using. Mix together until combined. Shape mixture into 2-3 inch meatballs and place them on the prepared baking sheet. Bake for 20-25 minutes until cooked through and browned.
Meanwhile, make the sauce by heating the olive oil in a 12-inch cast iron skillet over medium heat. Add garlic and pepper flakes, stirring, one minute, or until fragrant but without browning the garlic. Add tomato sauce to the pan, followed by sugar and salt. Turn heat to medium-low and allow to simmer until the meatballs are finished baking.
When the meatballs are done, add them to the skillet with the sauce. Spoon the sauce over the meatballs to coat.
!For the Buns:
Turn the oven up to broil. In a small bowl, combine the garlic, olive oil, and butter. Brush the garlic butter mixture onto the insides of the buns and broil until toasted and browned. Remove from oven.
Place 2 or 3 meatballs onto each toasted bun. Spoon some additional sauce over top of the meatballs. Top with shredded mozzarella and return the sandwiches to the broiler for another minute or two, until the cheese is melted and slightly browned. Remove from oven and sprinkle with fresh basil. Serve promptly.