Combine mayonnaise and Greek yogurt in a small bowl until smooth.

Stir sweet chili sauce and sriracha into the sauce.

Mix sesame seeds, cashews, and 1 tablespoon of scallions in the second small bowl.

Set the sesame-cashew crunch aside.

Add yellowtail tuna, remaining scallions, avocado, and salt to the mixing bowl.

Fold three-quarters of the sauce into the tuna mixture until creamy.

Slice the English cucumber lengthwise into thin ribbons using the mandoline slicer.

Overlap half of the cucumber ribbons into a rectangle on the cutting board.

Pat the cucumber rectangle dry with a paper towel.

Place half of the shredded carrots near one long edge of the cucumber rectangle.

Spoon half of the tuna mixture over the carrots.

Roll the cucumber tightly around the filling to form a cylinder.

Repeat the assembly and rolling process with the remaining cucumber, carrots, and tuna mixture.

Slice both cucumber rolls into sushi-style pieces with a knife.

Arrange the roll pieces on serving plates.

Top each serving with reserved sauce and sesame-cashew crunch.

Serve with soy sauce for dipping.