With a serrated knife, carefully slice each sub roll horizontally without slicing all the way through. Make sure the “hinge” of the bread remains intact and the roll doesn’t tear into two pieces.
Spread yellow mustard generously on the top and bottom interior surfaces of each bun. Then layer the remaining ingredients on each bottom bun in the following order: 4 pieces of ham, 4 pieces of bologna, 4 pieces of salami, 3 pieces of cheese, a few onion slices, a few tomato slices, and 1/2 cup of lettuce.
Close the subs and gently press them to compact the fillings. Wrap each sandwich tightly in plastic wrap or foil.