Preheat oven to 375 degrees F. In a food processor, process one of the buns until coarse crumbs form. Measure 3/4 cup of the crumbs; set aside remaining crumbs for another use.
In bowl combine egg, 1 cup of the tomatoes, the 3/4 cup bread crumbs, and basil. Add beef and mix well. Divide into four portions and shape each into a 4-inch square. Place on unheated rack of broiler pan. Bake 20 minutes (160 degrees F). Top with cheese. Bake 2 minutes more. Serve meat loaves on buns, with spinach and remaining tomatoes. Makes 4 sandwiches.