01 - Preheat oven to 400°F (200°C) and line a baking tray with parchment paper.
02 - In a large bowl, combine ground turkey, breadcrumbs, egg, minced garlic, parsley, Parmesan, onion powder, salt, and black pepper. Mix until ingredients are just combined.
03 - Form mixture into 16 uniform meatballs and arrange them evenly on the prepared baking tray. Bake for 18 to 20 minutes until cooked through and golden on the outside.
04 - While meatballs bake, combine maple syrup, Dijon mustard, soy sauce, apple cider vinegar, and smoked paprika in a small saucepan over medium heat. Whisk and simmer for 3 to 4 minutes until the glaze is slightly thickened.
05 - Transfer the baked meatballs to the saucepan and gently toss until they are evenly coated with the warm maple glaze.
06 - Slice the sub rolls lengthwise and spread the insides with unsalted butter. Toast in a skillet or in the oven until golden brown.
07 - Place four glazed meatballs onto each sub roll. Top each with a slice of cheese and return to the oven for 2 to 3 minutes, just until the cheese melts.
08 - Add a generous handful of baby arugula or spinach to each sub. Serve immediately while warm.