Pat the tuna steak dry with paper towels and season both sides with sea salt and black pepper.
Heat the sesame oil in a cast-iron skillet over high heat until shimmering.
Sear the tuna for 60 seconds on each side until a golden crust forms while the center remains rare.
Transfer the tuna to a cutting board and slice into long, thin strips.
In a small bowl, gently fold the rice vinegar into the cooked sushi rice.
Place the nori sheet on a bamboo rolling mat and spread the rice evenly over the bottom two-thirds of the sheet.
Arrange the tuna strips, julienned cucumber, and avocado slices in a horizontal line across the center of the rice.
Roll the nori tightly from the bottom up using the mat to maintain a firm shape, then seal the top edge with a drop of water.
Slice the roll into 8 pieces using a sharp, wet knife and sprinkle with sesame seeds before serving with tamari.