Fry bacon in skillet until crisp.
Remove from pan and crumble.
Drain off all but 2 tablespoons drippings and add onion and brown lightly.
Stir in cornstarch, salt, and dry mustard.
Drain beans, reserving 1/2 cup liquid.
Stir liquid into skillet and cook, stirring until mixture starts to boil.
Blend in brown sugar and vinegar.
Add green beans and heat thoroughly.
Turn into serving dish and garnish with egg :and crumbled bacon.