Make pavlovas: Heat oven to 250°F. Line 2 large baking sheets with parchment paper. In medium bowl, combine superfine sugar and cornstarch.
Using electric mixer, beat egg whites in large bowl until soft peaks form. Gradually add sugar mixture, 1 tablespoon at a time, beating until incorporated, then beat on high until whites are stiff and glossy; beat in vanilla and vinegar.
Use 4 small dabs meringue to stick parchment to baking sheets. Using spatula, spoon remaining meringue in 8 mounds onto prepared sheets. Spread each into 3-inch disk; then, using back of spoon, make deep indent in center of each.
Transfer to oven; bake 30 minutes. Turn off oven (do not open door) and let cool inside oven 1 ½ hours.
Meanwhile, make compote: In medium saucepan, combine rhubarb, strawberries, granulated sugar, orange zest, and 2 tablespoons water. Simmer, stirring occasionally, until rhubarb has broken down and mixture has thickened, 35 to 45 minutes; let cool.
When ready to serve, whip cream until soft peaks form. Fill meringue nests with cream, then top with compote, strawberries and pistachios, if desired.
