In a large bowl, mix together the egg, breadcrumbs, milk and Parmesan and set aside.
Heat the oil in a skillet over medium-high heat and add the onion and a pinch of salt. Cook, stirring, until softened, 5 to 7 minutes. Add the garlic and fennel seeds and cook for another minute. Set aside to cool.
Add the ground beef and ground pork to a medium bowl, spreading it out, and sprinkle the surface evenly with 1 teaspoon salt, a few turns of black pepper, a good pinch of crushed red pepper, the oregano and the parsley. Add the cooled onion mixture to the meat mixture. Mix with your hands quickly until just combined (don't overmix). Divide into 6 parts and shape into balls. Sprinkle with salt and pepper.
Heat a large cast-iron skillet or a griddle over medium heat and add a thin layer of oil. Cook the meatballs on one side until a brown crust is formed, 1 to 2 minutes, then flip and cook for 2 minutes; then smash with a big metal spatula. Top each with a slice of cheese and cook until the cheese is melted and the internal temperature is 160 degrees F, about 5 more minutes.
Toast the rolls or focaccia either in a skillet or the oven, then spread each side with marinara sauce. Top each with a patty and 2 big leaves of basil. Enjoy!