Preheat the oven to 325°. Line a baking sheet with parchment paper. Spread the pistachios in a pie plate and toast for 5 minutes, or until lightly browned. Let cool, then finely chop.
In a medium bowl, using a handheld electric mixer, beat the egg whites at medium-high speed until soft peaks form. Add the superfine sugar, 1 tablespoon at a time, beating well between additions. Increase the speed to high and beat until the whites are stiff and glossy, about 4 minutes. Beat in the lemon juice, cornstarch and vanilla. Fold in all but 1/4 cup of the pistachios. Spoon the meringue onto the baking sheet in 8 evenly spaced mounds. Using a small spoon or offset metal spatula, make a deep well in the center of each meringue. Sprinkle 2 tablespoons of the remaining pistachios over the tops.
Bake the meringues in the center of the oven for 5 minutes. Reduce the oven temperature to 225° and bake for 1 hour. Turn the oven off, prop the door slightly ajar and leave the meringues in for 1 hour longer. They should be white, crisp on the outside and chewy on the inside.
Using a sharp knife, peel the oranges, removing all the bitter white pith. Thinly slice the oranges crosswise. Set the meringues on plates. Top with the fresh orange slices and a scoop of the sorbet. Sprinkle with the remaining 2 tablespoons of pistachios. Garnish with the candied orange slices and serve.
The meringues can be stored overnight in an airtight container.