How to Make It
Preheat the oven to 250° and line 2 medium baking sheets with parchment paper. Position racks in the lower and upper thirds of the oven. In the bowl of a standing electric mixer fitted with the whisk, beat the egg whites with the cream of tartar at medium speed until soft peaks form. Slowly pour in 1/2 cup of the superfine sugar and beat at high speed until the egg whites are stiff and glossy. In a small bowl, whisk the sifted confectioners’ sugar with the remaining 1 tablespoon of superfine sugar. Using a rubber spatula, gently fold the sugar mixture into the meringue in 3 additions.
Spoon 3 rounded 1/2-cup mounds of meringue onto each baking sheet. Using the back of the spoon, form the meringue into 4-inch rounds with a well in the center. Place the meringues in the oven and immediately lower the temperature to 200°. Bake for 2 hours, or until crisp on the outside but still a bit chewy in the center; rotate the sheets halfway through baking. Let cool completely.
In a medium saucepan, heat the milk just until small bubbles appear around the edge. Add the tea bag, cover and let steep off the heat for 10 minutes, pressing the tea bag occasionally. Discard the tea bag.
In a medium heatproof bowl, whisk the egg with the granulated sugar and cornstarch. Gradually whisk in the warm milk. Transfer the mixture to the saucepan and cook over moderate heat, whisking vigorously, until very thick, smooth and shiny, 3 to 4 minutes. Scrape the pastry cream into a bowl and let cool slightly. Whisk in the cubed butter until incorporated. Place a sheet of plastic wrap directly onto the surface of the pastry cream and refrigerate for about 2 hours, until chilled.
Gently spread the pastry cream into the center of the meringues and arrange the nectarine slices on top in concentric circles, forming rosettes. Serve the Pavlovas right away.
The pastry cream can be refrigerated for 2 days.