Slice off top third of bread loaf; set top aside. Hollow out bottom section, leaving a 1/2-inch shell. (Reserve crumbs for another use). Combine mayonnaise, Italian seasoning, and pepper. Brush inside of bread shell with half of mixture. Set shell and remaining mixture aside. Cook onion, pepper strips, and celery in olive oil in a large skillet over medium-high heat until tender, stirring often. Arrange half of ham in bread shell and top with half of vegetable mixture, sprinkle with half of cheese. Spread remaining mayonnaise mixture over cheese. Repeat layers with remaining vegetable mixture, ham, and cheese. Replace bread top. Wrap sandwich in heavy-duty aluminum foil. Bake at 400 degrees for 30 minutes or until thoroughly heated. Cut sandwich into wedges, and serve immediately.