Soak panko in milk 5 minutes. Stir in eggs, feta, olives, peppers, onion, garlic, oregano, salt, and black pepper.
Fold in beef and lamb just until combined. Shape 9-by-5-inch loaf about 2 1/2 inches high on parchment-lined metal tray.
Before baking, promptly freeze raw loaf until firm. Wrap tightly, bag, label, and freeze up to 2 months. Keep oil, lemon juice, and reheating broth separate.
Thaw wrapped loaf completely on rimmed tray in refrigerator 24 to 48 hours. Unwrap fully; place on parchment-lined rimmed sheet.
Heat oven to 375°F. Mix oil and lemon juice; brush loaf. Bake 60 to 70 minutes to 160°F in center.
Rest 10 minutes; cut 8 slices. Refrigerate cooked leftovers within 2 hours.
Reheat cooked slices covered with a little reserved broth at 350°F for 20 to 25 minutes to 165°F in centers.