Heat oven to 350°F. Spray 9x5-inch loaf pan with cooking spray or grease with shortening. In large bowl, mix ground beef, 1/2 cup of the tomato sauce, the egg, bread crumbs, garlic powder and pepper. Pat half of meat mixture in pan.
Sprinkle 1/2 cup of the cheese over meat mixture. Spread spinach over cheese; sprinkle with remaining 1/2 cup cheese. Pat remaining meat mixture on top of cheese. Insert ovenproof meat thermometer so tip is in center of loaf.
Bake uncovered 1 hour to 1 hour 15 minutes or until thoroughly cooked in center and thermometer reads 160°F. Run knife around edges of pan. Place serving platter upside down over pan; turn platter and pan over. Remove pan.
In small microwavable bowl, microwave remaining tomato sauce, loosely covered, on High 30 to 60 seconds or until hot; pour over top of meat loaf.