Preheat the oven to 250ºF. Line a baking sheet with parchment paper.

Combine the egg whites and salt in the bowl of a mixer fitted with the whisk attachment. Beat on high speed until it starts to form peaks. With the mixer running, gradually add ¾ cup of sugar. Continue to beat on high until the egg whites are stiff and glossy, about 3 minutes.

Drop the egg whites into 6 mounds on the prepared baking sheet. With the back of a soup spoon, spread the egg whites into circles and create a well in the center with the edges higher than the centers. Bake the nests for 1–1½ hours (depending on humidity) or until the nests are crispy on the outside.

Put the strawberries in a blender. Grate the zest of the lemon into the blender with a handheld rasp grater (such as a Microplane) and squeeze in the juice. Add ¼ cup sugar and blend until smooth. Pour into a saucepan. Add the rhubarb and cook over medium heat for about 5 minutes or until the rhubarb is soft but not mushy. Let cool.

Whip the cream with the vanilla until peaks form. To serve, place a dollop of whipped cream in the center of each nest, top with some strawberry rhubarb mixture, and sprinkle with some chopped pistachios (if using).