In a small bowl mix together the white miso, ginger, vinegar, sesame oil and pinch of salt.
Cube the tuna and dice the 1/2 of an avocado. Transfer the tuna and avocado to a small bowl. Drizzle in the dressing and gently toss until evenly coated.
Slice your boule or baguette and place the tuna and avocado mixture atop the bread. Garnish with a sprinkling of black sesame seeds.