Preheat oven to 400. Melt butter and saute onions, green peppers, mushrooms and minced garlic.
Cut the French loaf lengthwise so to make one very long sandwich. Add the shaved roast beef. Top with sauted vegetables, top with swiss cheese. Spread the margarine on the top half of the loaf and sprinkle with garlic salt. Bake at 400 for 10-15 minutes, till cheese is melted and slightly browned. (You will probably have to remove the top half first, after it is just toasted. Meanwhile, make au jus according to package directions to dip sandwich in.