In a mixing bowl, combine the diced tuna, avocado, soy sauce, sesame oil, lime juice, grated ginger, and chopped scallions. Gently mix to combine without mashing the avocado.

Using a ring mold or a small bowl, pack the tuna mixture tightly and invert it onto serving plates. Remove the mold carefully.

Garnish with sesame seeds and additional avocado slices. Serve immediately with cucumber slices on the side.