Cut bread in half lengthwise. Place the halves cut-side up onto a large piece of parchment paper.
In a small bowl, whisk mayonnaise and mustard to combine. Spread mixture evenly across both halves of bread.
Layer lettuce, tomatoes, onions, and pickles on the top half of the bread.
Layer the cheeses and meats on the bottom half of the bread. Put the two halves together.
Wrap the finished sandwich tightly in parchment paper. Store sandwich like this in the refrigerator up to 4 hours.
When ready to serve, slice sandwich while still wrapped in the paper. Serve immediately.