Preheat your oven to 350°F (175°C) and grease your tart tins lightly. In a food processor, combine pistachios, almond flour, granulated sugar, and kosher salt, pulsing until finely ground. Add the whole egg and blend until a cohesive dough forms. Press the dough evenly into the tart tins, then bake for 15-18 minutes, rotating halfway through, until golden brown.

While the tart shells are baking, chop the fresh rhubarb into small pieces. In a medium saucepan, mix together granulated sugar and water to create a wet sand-like texture, then heat over medium. Once it begins to bubble, add the rhubarb and stir until the sugar dissolves. Incorporate orange zest, juice, ground cardamom, and any optional rosewater, cooking until minimal juice remains. Allow the compote to cool.

For the meringue, place egg whites and sugar in a mixing bowl set over a pot of simmering water, stirring continuously until the sugar completely dissolves and the mixture feels warm to the touch. Transfer to a stand mixer and whip on high speed until stiff peaks form and the meringue is glossy. Keep the Swiss meringue on low speed until ready to assemble the tarts.

Once the tart shells have cooled, fill each shell generously with the rhubarb compote, creating an even layer. Using a piping bag, pipe the prepared Swiss meringue on top of each tart, creating a decorative swirl or peaks. Using a kitchen torch, carefully torch the meringue until it turns golden brown, which adds a beautiful finish to your Pistachio Rhubarb Tarts with Rose.

To elevate the presentation, scatter some chopped pistachios over the toasted meringue. Serve the tarts at room temperature as a delightful spring dessert.