Warm stock in a medium saucepot and preheat oven to 400F. 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.
Meanwhile, melt butter in a saucepot over medium to medium-high heat and add pasta. Brown until nutty and fragrant, and deep golden in color. Stir in a few ladles of warm stock and allow it to almost cook away before adding more, like preparing risotto. Keep adding stock a little at a time until its all incorporated and pasta is al dente. Stir in lemon juice and feta cheese.
Serve orzo in shallow bowls topped with meatballs.