Cook the brown rice according to package instructions and allow it to cool to room temperature.
Slice the sashimi-grade ahi tuna into long, thin strips approximately half an inch thick.
Julienne the cucumber into thin matchsticks and thinly slice the radishes and avocado.
Lay a nori sheet on a bamboo sushi mat and spread half of the cooled rice evenly over the bottom two-thirds of the sheet.
Place half of the tuna, avocado, cucumber, and radish in a horizontal line across the center of the rice.
Grate the fresh ginger over the fillings and drizzle with half of the sesame oil, then season with sea salt and black pepper.
Roll the nori tightly from the bottom using the mat, applying gentle pressure to ensure a firm roll, and seal the top edge with a small amount of water.
Repeat the process for the second roll, then slice each into eight pieces using a sharp, damp knife and serve with tamari.