From the recipe I was taught while working at the deli.

cost:£1 serves:1
a classic sandwich, simplified

1 crusty bread roll
1 T yellow mustard
1 very thin slice of red onion
2 very thin slice of green pepper
2 slices pastrami
2 slices roast beef
1 slice provolone, muenster, or Port Salut cheese

Stack in order. It’s important that the vegetables are cut as thin as possible to really impart flavour without overwhelming the sandwich. You can warm the meats if you choose, but I like to do this sandwich as-is. Serve with kettle crisps.