Preheat oven to 325°F (165°C). Place a large roasting pan on the middle rack and fill it halfway with hot water to create a water bath.

Make the caramel by combining 1 cup sugar and 1/4 cup water in a medium saucepan over medium heat. Swirl the pan occasionally without stirring until the mixture turns deep amber, about 8-10 minutes. Immediately pour into a 9-inch round cake pan or bundt pan, tilting to coat the bottom evenly. Set aside to harden.

Grind the toasted pistachios in a food processor until they form a smooth paste, about 2-3 minutes, scraping down the sides as needed.

In a blender, combine the pistachio paste, sweetened condensed milk, evaporated milk, whole milk, eggs, egg yolks, vanilla extract, and salt. Blend on low speed for 30 seconds until just combined and smooth.

Strain the custard mixture through a fine-mesh sieve into the caramel-coated pan to remove any lumps. Cover the pan tightly with aluminum foil.

Carefully place the covered pan into the water bath in the oven. Bake for 55-65 minutes until the flan is set but still jiggles slightly in the center when gently shaken.

Remove from the water bath and let cool to room temperature, about 2 hours. Refrigerate uncovered for at least 8 hours or overnight.

To unmold, run a thin knife around the edges of the pan. Place a large rimmed serving plate over the pan and flip confidently in one motion. Let sit for 2-3 minutes if needed, then lift the pan away. Garnish with chopped pistachios before serving.