Slice beef into thin pieces, using my electric knife works awesome for this job.
Place roast beef slices into roasting pot, pour in left-over au jus or prepared gravy, mix together and warm up in oven for 30 mins. at 300 degrees.
If your au jus is too thin, thicken with a bit of gravy thickener, before adding beef.
Turn temperature to low and keep warm until ready to use.
Prepare onion rings while the beef warms up.
Heat the oil in a deep pot (you can also use a deep fryer)
Separate the onion slices into rings, and set aside.
In a small bowl, stir together the flour, baking powder and salt.
Dip the onion slices into the flour mixture to coat; set aside.
Whisk the egg and milk into the flour mixture using a fork.
Dip the floured rings into the batter to coat.Then dip into the bread crumbs
Fry in oil until golden brown 2-3 minutes
Remove to paper towels to drain.
Assemble by cutting thick slice of French bread, top with slices of roast beef, cover with gravy and top with onion rings.