Combine the ground pork and beef with the bread crumbs, Romano cheese, parsley, garlic, beaten egg, salt, and pepper in a large mixing bowl.
Slowly pour in the milk or water, then gently mix everything together by hand until just combined to avoid tough meatballs.
Scoop out the meat mixture using a small cookie scoop or large spoon onto a parchment-lined tray.
Roll each portion into golf ball-sized meatballs without overworking the meat.
If the meatballs feel too soft to hold their shape, chill them in the fridge while heating the oil.
Heat enough frying oil in a 10-inch skillet to a depth of 1-2 inches over medium-high heat.
Arrange the meatballs in the hot oil without crowding the pan; cook in batches if needed.
Fry the meatballs for about 2 to 3 minutes per side until evenly browned, about 6 minutes total.
Remove the meatballs and drain excess oil from the skillet, then return it to medium heat.
Pour in the marinara sauce and pesto sauce, stir gently, and add the meatballs back to the skillet.
Simmer the meatballs in the sauce for 15 minutes until cooked through and the sauce is hot.
Lower the heat to a gentle simmer and nestle slices of mozzarella between the meatballs.
Allow the cheese to soften and melt for 3 to 5 minutes over low heat.
Optional: Place the skillet under a preheated broiler for about 5 minutes until the cheese and sauce are bubbling lightly on top.