In a small bowl, stir together the mayonnaise and Dijon mustard until smooth to make a tangy mustard mayo.
Spread this mixture evenly on one side of each rye bread slice (about 1/4 per slice).
Place two slices of bread (spread side up) on a board. Layer each with: 1 slice cheese, 2 slices pastrami, another slice cheese, a few tomato slices, and lettuce. Add a pinch of salt and pepper if you like.
Top with the remaining bread slices (spread side down), press gently, then cut each sandwich in half. Pack in a lunchbox and enjoy!
Notes
Nutrition per serving: Calories: 623, Total Carbs: 29g, Net Carbs: 24g, Fats: 37g, Protein: 42g.