Heat oven to 375°F. In medium bowl, mix ground beef, bread crumbs, onion, ketchup, mustard and salt. Press mixture in bottom and up sides of ungreased 9-inch pie plate. Bake 15 minutes.
In medium saucepan, place water, butter and garlic salt. Heat to boiling. Remove from heat. Add milk. With fork, stir in potato flakes. Stir in 1/2 cup of the cheese.
Remove partially baked beef crust from oven; pour off any drippings. Spoon potato mixture evenly into crust. Return to oven; bake 10 to 15 minutes longer or until beef is thoroughly cooked and thermometer placed in center of crust reads 160°F, and potatoes are hot.
Remove pie from oven. Top with tomato, remaining 1/2 cup cheese and crumbled bacon; bake 5 minutes longer or until cheese is melted. Remove from oven; top with green onions. Let stand 10 minutes before serving.