1Combine sesame seeds, brown sugar, and garlic powder on a plate. Press the tuna steak into the mixture until fully coated.
2Heat a large pan over medium-high heat and add high heat oil. Sear one side of the tuna for about 1 minute, flip and repeat until golden brown.
3In a small bowl, whisk together soy sauce, sriracha, and rice vinegar.
4Slice the cooled tuna steak into strips using a very sharp knife.
5Serve the seared tuna over sushi rice with sliced cucumbers and half an avocado. Drizzle the sauce on top or serve on the side and garnish with chives or scallions.