This is a recipe from Nigella Lawson, designed to use leftover Texas brisket and gravy from her Texas Brisket recipe, which I also have posted. It's a loose recipe in terms of amounts - leaving it up to you how much meat and gravy you want to use for your leftover sandwich. Have fun with it!
Reheat leftover brisket slices in gravy, in an ovenproof dish covered tightly with foil, either in a preheated to 350 degree for 20-30 minutes or in a microwave (cover it with plastic wrap instead of aluminum foil) in 30 second bursts until hot.
Cut 2 thick slices of bread; dip 1 slice in gravy and sit wet side up on a plate; sit meat on top then add dollop of horseradish sauce; using remaining piece of bread, wipe out gravy dish and sit the sopping slice of bread, wet side down on top of beef.
You might want to eat this one with a fork and lots of napkins!