Deli sandwiches work because they are built with intention. Use good bread, season your spreads and add something sharp. Pickles are non-negotiable.
Start with a toasted bagel or rye. Add a spread, then meat or cheese, then crunch, then pickle. Spicy silverskin onions with pastrami and cream cheese is an easy win. Gherkins with tuna is another.
The trick is not more ingredients. It is better structure and one jar that brings bite.