Preheat a grill or panini maker. Brush the onion slices on both sides with the olive oil. Sprinkle with the salt and pepper. Grill the onion slices until tender and lightly browned, 3 to 4 minutes per side. Set aside to cool.
Pull out and remove some of the bread from each half of the rolls. Spoon half of the sauce over the bottom halves of the rolls. Sprinkle with red pepper flakes if using. Add the arugula, meatballs, prosciutto slices, grilled onions and the remaining sauce. Place 2 slices of cheese on top. Add the tops of the rolls and drizzle with olive oil. Using a pastry brush, brush the oil evenly over the tops.
Place the sandwiches in a panini maker and cook until the cheese has melted and the meatballs are heated through, about 5 minutes.
Cut the panini in half and serve.
Preheat the oven to 400 degrees F.
In a large bowl, combine the breadcrumbs, Parmesan, basil, parsley, milk, tomato paste, salt, pepper, eggs, garlic and onions. Using a wooden spoon, stir to blend. Add the ground turkey and sausage meat. Using fingers, gently mix all the ingredients until thoroughly combined.
Using a 1-ounce cookie scoop or a small ice cream scoop, form the meat mixture into 1 1/2-inch balls (22 to 24 meatballs). Place the meatballs on a heavy nonstick baking sheet, spacing apart. Drizzle with olive oil and bake until cooked through, 15 minutes. Yield: 22 to 24 meatballs.