Cut a thin slice off the top of bread; set top aside. Hollow out the loaf, leaving a 1/4-in. shell. Dice removed bread and set aside.
In a nonstick skillet, cook beef over medium heat until no longer pink; drain. Add the onion, green pepper, spinach and tomato; cook 3-4 minutes longer or until vegetables are crisp-tender. Stir in the oregano, salt, pepper, and diced bread.
Spoon into bread shell; replace top. Wrap tightly in heavy-duty foil. Bake at 400° for 20 minutes or until heated through. Cut into slices. Yield: 6-8 servings.