LES of NEW YORK
Pastrami is descended from another form of this ancient jerky, known as basturma. The 14th-century Ottomans pressed their slices of fish and meat to extract any moisture, rubbed them with a fenugreek-heavy mixture of spices, and left them to air-dry. The preserved protein was especially useful sustenance for Ottoman army troops marching long distances. When the troops and their jerky eventually reached the Balkans, the Romanian Jews of the area adopted the preservation method and added their own local spices; the altered concoction was commonly referred to as pastrama (though other spellings and pronunciations, such as pastirma and pastromaabounded).
PASTRAMI on RYE
Modern delis all use the same general process for their pastrami: start with brine-cured beef, rub it down with red wine vinegar, add a centuries-old secret family spice mixture (which may include any and all of the following: peppercorns, allspice, bay leaves, cinnamon, cloves, coriander, ginger, juniper berries, garlic, red pepper flakes, mustard seed, cardamom, and onion), optionally dry-cure for up to two weeks, smoke for seven hours, and braise or steam.
The pastrami sandwich is famous for this particularly staunch proscription, like a chef's tasting menu that stipulates "no substitutions": cheese, barbecue sauce, gravy, white bread, lettuce, tomato, and most importantly, mayo, are not acceptable on a pastrami sandwich.