Rhubarb and Pistachio Pavlova

Preheat the oven to 200 (F). Prepare a large baking sheet by lining with parchment paper.
In a small bowl, whisk together 1 cup granulated sugar and 1 tablespoon cornstarch. Set aside.
In the bowl of a freestanding electric mixer fitted with a whisk attachment, whisk together 4 egg whites, 1 teaspoon lemon juice, and 1/4 teaspoon kosher salt on low speed until very frothy.
When the mixture is frothy, increase mixer speed to medium-high and gradually add the sugar and cornstarch mixture (from the second step), beating until stiff, glossy peaks form, about 12 to 15 minutes. Be sure to scrape down the sides of the bowl with a rubber spatula halfway through the mixing process.
When the mixture is glossy and stiff, use a rubber spatula to mound onto the center of your prepared parchment sheet. Use the rubber spatula to spread the mound until it is a 7-inch circle, and create a well 3 1/2-inches wide and 1-inch deep in the center of the mound.
Transfer to the preheated oven and bake for 2 1/2 hours, or until the mound is crisp on the outside and soft inside.
When the pavlova is ready, turn off the oven heat and let the pavlova cool in the oven for at least 2 1/2 hours, or until completely cool (preferably overnight).

Preheat the oven to 400 (F).
In a medium bowl, toss together 1 pound sliced rhubarb, 1/4 cup water, 3/4 cup granulated sugar, 1 tablespoon lemon juice, and 1/8 teaspoon salt.
When the rhubarb is covered, transfer to a 9 by 13-inch glass baking dish and bake until just tender, 10 to 12 minutes.
Use a rubber spatula to carefully transfer the rhubarb pieces (they will be very soft) to a parchment-lined rimmed baking sheet; reserve the juices. Allow both the rhubarb and juices to cool completely.
When the rhubarb has cooled and you're ready to serve your pavlova, make the vanilla whipped cream by combining 1 1/2 cups heavy cream and 1 teaspoon vanilla extract in the bowl of a freestanding electric mixer fitted with a whisk attachment. Whisk on medium-high speed until soft peaks form and you have whipped cream.
Once the whipped cream is ready, transfer to the pavlova well. Top with rhubarb and garnish with 1/8 cup pistachios. Spoon over rhubarb juices and serve immediately.

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