In a bowl, mix tuna, Greek yogurt, 1.5T lemon juice, 1T allulose, sweet corn, and pepper together.
Place the heated and halved bread on a piece of plastic wrap, then pile all of the previously made mixture onto one side.
Top with julienned onions, julienned cucumber, sliced boiled egg, and halved cherry tomatoes.
In a bowl, mix 0.5T whole grain mustard, 0.5T allulose, and 0.5T mayonnaise to make a sauce. Spread it on the other piece of bread, cover the sandwich, and wrap it up.