In a large bowl, combine ground beef, bread crumbs, chili sauce, egg, salt, and freshly ground black pepper; mix well. Shape into an 8x4 inch loaf pan.
Place loaf pan on a rack in an oven-roasting pan. Bake for 1 hour, or until done. Cook until internal temperature measures 160 degrees F (70 degrees C); the meat should be well done, with no trace of pink. Remove from oven, and allow to rest for 10 minutes before serving.
While the meatloaf is resting, prepare the gravy. In a medium saucepan, melt butter or margarine over medium heat. Add mushrooms and onions; cook and stir until tender. Add beef broth; simmer for 5 minutes, stirring occasionally. Combine water and cornstarch in a small cup; stir into broth. Cook and stir for 1 minute, or until thickened.