You can use different types of bread!
In a bowl, mix tuna, Greek yogurt, 1.5T lemon juice, 1T allulose, sweet corn, and pepper together.
On top of a sheet of plastic wrap, place bread that has been warmed and sliced in half. Stack all of the tuna mixture on one side of the bread.
Top with julienned onion, julienned cucumber, sliced boiled egg, and halved cherry tomatoes.
In a bowl, combine 0.5T whole grain mustard, 0.5T allulose, and 0.5T mayonnaise. Spread this sauce on the other slice of bread, close the sandwich, and wrap it with the plastic wrap.