Preheat oven to 350°F with rack in center position. Coat a 13-by-9-inch baking dish with cooking spray. Line with parchment paper, leaving 2 inches of overhang on long sides.

Process shelled pistachios, sugar, 2 tablespoons cornstarch, and 1/4 teaspoon kosher salt in a food processor until finely ground. Add butter; pulse until mixture comes together.

Transfer pistachio mixture to prepared baking dish. Using damp fingers or an offset spatula, spread mixture in an even layer on bottom and about 1/4 inch up sides of dish. Bake until lightly golden, 12 to 14 minutes. Let cool in baking dish on a wire rack for 20 minutes.

Zest and juice 5 lemons to yield 2 tablespoons zest and 1 cup juice. Place zest and juice in a large bowl. Whisk in eggs, condensed milk, and remaining 2 tablespoons cornstarch and 1/2 teaspoon kosher salt.

Pour lemon juice mixture over crust. Bake until filling is just set, 25 to 30 minutes. Let cool completely in baking dish on a wire rack, about 1 hour. Cover and refrigerate until firm, about 3 hours.

Sprinkle with chopped pistachios and flaky sea salt. Zest remaining lemon over top before slicing.