Preheat oven to 450 degrees F with rack at center. Line a baking sheet with parchment paper.
For the meatballs: Pour breadcrumbs in a small bowl, add a few grates of nutmeg and douse well with milk. Toss and let the milk absorb.
Place the meats in a large mixing bowl, season with salt and pepper, combine them and make a well. Rip off some parsley tops and finely chop; add to meats. Peel garlic. Grate in garlic with plane or chop by hand into meats. Pour in cheeses, sprinkle on fennel and red pepper flakes. Add egg and EVOO. Roll up sleeves, take off your jewelry. Combine the mixture and quarter the meat. Form each quarter portion into 2 large balls and place on parchment; roast 15 to 18 minutes.
For the sauce: Meanwhile, in a medium Dutch oven or a deep cast-iron skillet heat EVOO, 2 turns of the pan, and melt butter into oil. When it foams, grate in garlic and stir 1 minute. Add canned tomatoes and passata. Break up tomatoes with spoon and bring to a bubble. Stir in basil leaves. Reduce heat on sauce a bit and let it bubble away 15 minutes, breaking sauce up with a paddle or spoon occasionally. Add salt and sugar if needed.
For the garlic bread: While sauce and meatballs are cooking, heat EVOO, 2 turns of the pan, in a small pot or pan over medium heat and melt butter into it. Crush and grate garlic into butter and remove from heat. Split bread and add to oven a rack below the meatballs. Toast 5 minutes. Remove and brush with all the garlic and oil, top with cheese and chop a little parsley to sprinkle on top. Return to oven, warm another 3 minutes to set, remove.
Remove meatballs from oven, drop into sauce and let them roll!
Serve meatballs in rimmed plates or shallow bowls and add cut garlic bread for the mopping.
Non-Italian tomatoes are sometimes less sweet so the sauce may need a bit of sugar.