Slice the bread into even, medium slices.
Lightly toast the bread (in a toaster or a pan).
Spread equal amounts of the sauce on each of the slices.
Separate the layers of pastrami, lightly oil a non-stick frying pan and warm up the slices evenly (about 1-2 minutes each side) then place on one slice of bread.
Reheat the caramelised onions, move into a pile and place a slice of the cheese on to melt. Carefully transfer the onions and cheese on top of the pastrami slices.
Top with pickles and sauerkraut, followed by the second slice of bread. Chow down!