Slice off the top and bottom of the oranges, then peel all round the fruit down to the flesh. Using a sharp knife, cut out the segments between the white membranes. Arrange the orange segments, fennel and avocado on a platter or on 4 plates.
Combine the balsamic, oil, sugar, orange zest and juice.
Drizzle the vinaigrette over the salad, season with salt. Garnish with the pistachios and basil leaves.