Place a large nonstick skillet over mediumhigh heat; add onion. Coat onion with cooking spray, and sauté 5 minutes. Add garlic, and sauté 30 seconds. Remove from heat. Stir in spinach and next 3 ingredients.
Place steak between 2 sheets of heavyduty plastic wrap; flatten to an even thickness using a meat mallet or rolling pin. Spread spinach mixture over beef, leaving a 1/2-inch margin around outside edges. Roll up beef, jelly-roll fashion, starting with 1 short side. Secure roll at 2-inch intervals with heavy string.
Place beef on broiler pan coated with cooking spray. Sprinkle beef with pepper; coat with cooking spray. Bake, uncovered, at 425° for 10 minutes. Reduce heat to 375°. Bake 25 minutes or to desired degree of doneness. Let stand 10 minutes. Remove string, and cut beef into 6 slices.