Calabrian chili mayonnaise: in a medium bowl, add mayonnaise and Calabrian chilis and whisk to combine. Taste and add a pinch of ground black pepper if desired.
Store in a sealed container in the refrigerator for up to 2 weeks.
Sandwich assembly: slice the focaccia and toast it if you want. Toasting will crisp up the bread and keep it from tasting stale.
Spread some 'nduja on the bottom slice of bread, then top with drained giardiniera.
Layer on the slices of fresh mozzarella and then top that with slices of salami and soppressata.
Spread a bit of Calabrian mayonnaise on the top piece of bread and place it on top of the sandwich.
Serve and enjoy.