Heat oven to 400°F. Bake onion rings on ungreased large cookie sheet as directed on package.
Meanwhile, toast bread. Spread each toasted bread slice with mustard-mayonnaise sauce. Top each with beef; sprinkle with garlic-pepper blend. Top each with tomato slices.
Remove cookie sheet from oven. Push onion rings to one side of cookie sheet. Place sandwiches on cookie sheet; top each with cheese slice and onion rings.
Return to oven; bake an additional 6 to 8 minutes or until sandwiches are thoroughly heated and cheese is melted.