Set a steamer basket over 1 inch of simmering water and keep the water below the basket.

Loosely pile the pastrami in the basket, cover and steam for 4 to 6 minutes until supple and heated to 165°F.

Spread 1 tablespoon mustard on one side of each rye slice.

Divide the hot pastrami over 4 slices, folding rather than compressing it so steam can escape.

Close each sandwich with a second rye slice, mustard-side down, and cut it once through the center.