Heat oven to 200C/fan 180C/gas 6. Tip the pork, onion, half the parsley and the lemon zest into a food processor with lots of pepper and some salt if you wish, then pulse until well mixed.
Shape the meat into 25-30 balls and put in a large roasting tin. Arrange the mushrooms in a separate flame-proof roasting tin, sprinkle with garlic and drizzle with the oil, salt and pepper. Roast the meatballs for 25 mins, adding the mushrooms on the shelf below after 10 mins.
Transfer mushroom tin to hob. Stir in sherry and crème fraîche and bring to a gentle simmer. Stir in the meatballs, then spoon it all onto a large warm dish. Serve sprinkled with the remaining parsley, roughly chopped.