The pastrami dip takes the deli's most carefully steamed meat and then deliberately soaks its bread in jus. Hot pastrami is normally protected by thin rye whose only job is to stay out of the meat's way; the dip throws that logic out and piles the same brined, peppered, smoked, steamed pastrami onto a long roll that is dipped or ladled until the crumb is saturated. The defining move is the controlled drowning of the bread, which is the opposite instinct from the rye build. The sandwich is engineered to be eaten right at the edge of structural failure, and that edge is the point.
The craft is in the roll and the jus. The roll has to start sturdy, a long loaf with a crust firm enough to absorb that much liquid and still be liftable for one more bite, because a soft roll dissolves into the dip before it reaches the hand. The pastrami is hand-sliced against the grain, thick enough to have presence and tender enough to fold, then steamed slack so it drapes into the bread rather than sitting on it. The jus is seasoned to be the dominant flavor, since the saturated crumb will carry it through every bite, and it is the dip, not mustard, that does the work of cutting the rendered fat. Many builds serve the jus on the side or apply it to order so the eater controls how far past the edge to push, which is the same dial the French dip offers. Built right it is rich and barely contained, the roll holding a wet pile of spiced beef just long enough to finish it.
The pastrami dip belongs to the wet-roll family, where the close relations keep the soaked-bread logic and change the meat. The Los Angeles French dip serves roast beef with the jus in a cup for dipping. The Chicago Italian beef ladles seasoned jus over thin-sliced roast beef and adds giardiniera. Beef on weck swaps the roll for a salt-and-caraway kummelweck and leans on horseradish. Each of those is its own sandwich and deserves its own article rather than being crowded in here.