Preheat the oven to 400 degrees. In a medium saucepan, heat the chicken stock over low heat; keep warm. On a rimmed plate, soak the bread in the milk.
In a large bowl, combine the lamb and egg. Wring out any excess milk from the soaked bread and crumble the bread into the meat. Stir in 1/4 cup parsley, the mint, onion, garlic, lemon zest, oregano (or marjoram), 1/2 tsp. salt and 1/4 tsp. pepper. Drizzle with EVOO; mix. Roll the mixture into 20 meatballs and arrange on a nonstick or parchment-lined baking sheet. Bake until browned, 15 to 18 minutes.
Meanwhile,in a medium saucepan, melt the butter over medium heat. Add the orzo and cook until golden, about 5 minutes. Stir in a few ladlefuls of the warm stock and allow it to absorb before adding more. Keep adding stock a little at a time and cook until the orzo is al dente, about 20 minutes. Stir in the spinach to heat through in the last minute of cooking. Remove from the heat. Stir in the lemon juice and feta.
Serve the orzo in shallow bowls. Top with the meatballs and remaining 1/4 cup parsley.