In a mixing bowl, combine ground pork and beef. Add half of the finely chopped caramelized onions (save half for the sandwich).

To make panade add chopped baguette ends, and milk into a bowl and mix into a paste.

Add the panade, 1 egg, to the meat and season with salt and black pepper. Optionally, add pimenton, paprika, garlic, or rosemary for extra flavor. Mix the ingredients thoroughly for better texture and taste.

Shape the Meatballs: Roll the meatball mixture into evenly sized portions using parchment paper or aluminum foil. Roll each portion for a consistent shape.

Prepare a sauté pan with oil and sear the meatballs on high heat until browned all around. They will still be raw in the middle, as they will finish cooking in the tomato sauce.

While the meatballs are cooking, preheat the oven and set on broil. Cut your baguette into the desired sandwich size.

Add meatballs to the sauce and finish.

Add a thin layer of tomato sauce to the bread. Spread the remaining caramelized onions on the bottom. Place a few seared meatballs on the sandwich. Layer the mozzarella cheese slices on top.

Transfer the sandwich to a baking tray and place it in the oven under the broiler to melt the cheese and give it a golden color.

Once the sandwich is out of the oven, add a handful of fresh arugula. Serve and enjoy your tasty meatball sub!

Quick Tips: You can caramelize onions in advance and freeze them for future use. Cooked meatballs can be refrigerated and enjoyed for a few more days