How to Make It
Preheat the oven to 350°. Line a baking sheet with parchment paper. In a small bowl, toss the pistachios with the cornstarch.
In the bowl of a stand mixer fitted with the whisk, beat the egg whites with the salt at high speed until foamy, 2 minutes. Beat in the vinegar, then beat in the sugar, 1 tablespoon at a time, and continue beating until the whites are glossy and stiff peaks form, 8 to 10 minutes. Gently fold in the pistachio mixture. Using a large spoon, dollop the meringue onto the prepared sheet and spread into a 10-inch round with a slight indentation in the center. Lower the oven temperature to 225° and bake the meringue for about 1 1/2 hours, until crisp but still chewy on the inside. Turn the oven off; let the meringue rest in the oven for 1 hour. Transfer to a rack and let cool.
In a small saucepan, simmer the rhubarb, sugar, lemon zest and lemon juice over moderate heat, stirring and mashing the rhubarb with the back of a wooden spoon, until the sugar is dissolved and the rhubarb breaks down, about 5 minutes. Remove the saucepan from the heat and stir in the quartered strawberries and vanilla bean paste. Let cool completely.
In a large bowl, using a hand mixer, beat the cream with the mascarpone at medium speed until moderately firm, about 3 minutes. Stir 1/4 cup of the whipped cream into the cooled rhubarb, then fold the mixture into the remaining whipped cream. Spoon into the center of the meringue. Garnish with the small strawberries and chopped pistachios and serve.