In a medium bowl, mix together tuna, mayonnaise, mustard, soy sauce, rice vinegar, and garlic powder until well combined.
Stir in celery and carrots.
Lay a sheet of nori on a clean surface, shiny side down.
Place a handful of greens on the bottom third of the nori, then spoon the tuna mixture over it.
Top with a few avocado slices.
Roll the nori tightly from the bottom, like a sushi roll. Wet the edge to seal.
Slice in half and serve immediately, or wrap and chill until ready to eat.