Put the ground pork and beef into a large bowl along with the bread crumbs, Romano cheese, parsley, garlic, egg, salt, and pepper.
Slowly pour in the milk or water, and gently combine the ingredients using your hands until just mixed, taking care not to overwork the meat.
Scoop out portions of the mixture using a cookie scoop or large spoon onto a parchment-lined tray.
Shape each portion into a golf-ball size meatball using your hands, avoiding pressing too hard to keep them tender.
If the meatballs feel too soft, chill them in the refrigerator while you prepare to fry.
Heat olive oil or frying oil in a 10-inch skillet to a depth of 1-2 inches over medium-high heat.
Place the meatballs in the hot oil without crowding the pan; cook them in batches if necessary.
Fry each meatball for about 2-3 minutes per side, until golden brown and cooked on the outside, about 6 minutes total.
Remove excess oil from the skillet, then return it to the stove.
Pour in the marinara and pesto sauces, stirring to combine, then carefully add the meatballs back into the sauce.
Let the meatballs simmer in the sauce for approximately 15 minutes, ensuring they are cooked through with no pink inside.
Turn the heat to low and tuck mozzarella slices between the meatballs, allowing the cheese to soften and start melting for 3 to 5 minutes.
Optionally, broil the skillet for 5 minutes until the cheese and sauce bubble lightly on top before serving.
