A classic Reuben uses Thousand Island dressing, but throw some sharp mustard in here and you’ll get a great contrast to the rich meat and cheese. Instead of broiling or using a sandwich press, fry up the meat in a sauté pan, and then melt the cheese directly over the meat for maximum meat-cheese adherence.
Heat the oil in a pan over medium heat. Add the pastrami in 2 piles and heat to warm through and lightly brown the meat, 2 minutes. Lay 1 slice of cheese over each pile, and cover the pan with a lid until the cheese is completely melted, another 2 minutes.
Spread the mustard onto the bread, and then assemble the sandwiches, layering with sauerkraut, if using. Serve with pickles.