Warm stock in a medium saucepot and preheat oven to 400ºF. In a small bowl, soak bread in milk.
Place meat in a mixing bowl and add egg. Wring out excess milk from the bread and crumble into meat bowl. Season meat with salt, pepper, oregano or marjoram, mint, half the parsley, the onion, garlic, lemon zest and spinach. Drizzle EVOO once around the bowl and mix meat with your hands. Roll 20 meatballs and arrange them on a nonstick or parchment-lined baking sheet. Bake about 15-18 minutes until cooked through and browned.