Preheat oven to 400°F.
For the meatballs, soak bread in milk. Place meat in a bowl and add eggs, garlic, salt, pepper, cheese, nutmeg (grated over the bowl), parsley, mint and EVOO. Squeeze excess milk from bread and crumble with your fingers as you add it to meat. Mix well to combine.
For the sauce, heat a deep ovenproof saucepan or skillet over medium heat and add EVOO, 1 turn of the pan. Melt in butter then add onions and garlic, and sweat until very soft and tender, partially covering the pan and stirring occasionally. Add tomatoes and passata, stock, salt and tarragon, and simmer for 15 minutes.
Use a scoop to form 2 1/2-inch, even-sized meatballs and arrange them in the skillet in the sauce – the sauce should not cover the balls. Transfer pan to oven and roast 20-25 minutes until cooked through and the meat is lightly browned on top.
Serve with extra shaved Pecorino and charred bread rubbed with garlic, drizzled with EVOO and sprinkled with sea salt for dipping.