Add the pork and beef to a bowl and season liberally with salt and pepper. Trim the crust of bread and chop into fresh breadcrumbs in a food processor, scrape into a small bowl and soak in milk. Return the processor to the base. Squeeze out some of the liquid from the bread and add the moistened bread to the meat, season with a little freshly grated nutmeg, a few grates. Add a beaten egg yolk and half of the chopped parsley to the bowl, a couple of tablespoons. Add 2 cloves finely chopped garlic to the meat. Add the mortadella to the food processor and pulse to very finely chop the meat, add to the ground meats along with a couple of handfuls of grated pecorino. Drizzle the meat liberally with extra-virgin olive oil. Place a baking rack over a baking sheet or cover a baking sheet with parchment baking paper. Mix the meatballs and roll into large walnut-size balls and roast 12 to 15 minutes to light brown, but not cooked through.
Meanwhile, heat extra-virgin olive oil, 3 turns of the pot, over medium to medium-high heat in a large Dutch oven. To the warm oil, add fennel seeds, chile pepper, onions, and sliced garlic, season with salt and pepper, and marjoram. Stir 4 to 5 minutes then add tomato paste and stir 30 seconds, pour in wine and stir a minute, then add stock and tomatoes. Break up the tomatoes into small bits and when sauce comes to a boil reduce heat to simmer, add a few leaves of torn basil, reserving a few for garnish. Add the remaining chopped parsley and gently slide in the meatballs. Simmer the mixture 15 to 20 minutes to combine flavors and finish cooking the meat through.
Cook's Note: Cook the pasta at this time or if this is a make-ahead meal cool completely, cover and store. Reheat the sauce over medium heat covered until it bubbles then uncovered at gentle simmer while pasta cooks.
Cook the pasta in large pot of boiling, salted water to al dente. Save about 1/2 cup starchy cooking liquid just before draining. Add the drained pasta back to the hot pot with butter, a few ladles of sauce and the reserved starchy liquid. Toss to combine and melt butter. Serve in bowls with lots of sauce and meatballs on top, lots of shaved cheese and a few leaves of torn basil.
Serve with simple tossed greens salad if you like but it'll probably get ignored...it is spaghetti and meatballs night!