Place bread on sheet pan and toast in 350 degrees oven until crispy and dry; let cool, then pulse in processor until fine. Add cream to crumbs and set aside to soak. Meanwhile, add 2 T olive oil to skillet over medium-high heat and sauté onion and garlic until softened, approx. 3 min. Season with 1/2 t salt, then add mixture to bread crumbs; chill.
Preheat oven to 350 degrees. In large mixing bowl, combine ground meats, finely chopped pancetta (it is easiest to chop if previously sliced very thin and nearly frozen), eggs, chilled bread crumb/onion mixture, parsley, Parmesan, 1 t salt, pepper and chili flakes. Shape into balls (larger than golf balls, but smaller than oranges for best results). Add remaining olive oil to large skillet and sear meatballs, a few at a time, on all sides over medium heat.
Transfer to baking dish, packing tightly in a single layer. Cover with marinara sauce, then bake until 155 degrees internal temp is reached, approx. 20 min, depending on the size of meatballs. Top with shredded mozzarella and return to oven until melted and bubbling and internal temp reaches 165 degrees. Serve with pasta, bread and wine, as desired.