Peel and section the grapefruit, and remove the membranes and seeds. Cut the fruit into triangular wedges. Cover and chill.
Mix 1/4 cup of the lime juice, the zest and soy sauce in a small bowl.
Just before serving, peel, halve and pit the avocados. Place the halves, cut side down, on a cutting board and slice them into wedges the same size as the grapefruit. Sprinkle them with the remaining lime juice to prevent browning.
For each serving, place a shiso leaf on a small plate or bowl. Pile on avocado and grapefruit wedges in overlapping fashion to form a mound. In a deep bowl, the food should come to just below the edge; in a shallow bowl or plate, the arrangement should rise just above the rim. Leave empty space on all sides of the mounded food. Pour the dressing on the side to form a small pool. Garnish with nasturtium flowers