Preheat the oven to 350 degrees . Spread the pistachios on a baking sheet and bake until lightly toasted, 6 to 7 minutes; let cool. Transfer to a food processor and coarsely grind; transfer to a medium bowl and toss with 1/2 tsp. salt. Mix in the marshmallow cream. Using damp hands (moistening them again as needed), press the mixture evenly into a 9-inch pie pan to form a crust.
In a small bowl, sprinkle the gelatin over 1 tbsp. cold water.
In a large, heavy saucepan, whisk the sugar, lemon zest, lemon juice and eggs. Cook over low heat, whisking constantly, until the mixture thickens but does not simmer, about 8 minutes. Immediately remove the pan from the heat; whisk in the gelatin and 4 tbsp. butter until absorbed. Whisk in the remaining 12 tbsp. butter and 1/2 tsp. salt until the mixture is smooth. Transfer the lemon curd to a container; refrigerate for a few hours or overnight.
Remove the curd from the refrigerator and spread evenly onto the crust. Freeze for 1 hour.
Preheat the broiler. Remove the pie and top with the large marshmallows; fill in the cracks with the mini marshmallows, Broil until golden-brown on the top, about 3 minutes. Remove and let cool slightly; slice with a wet knife.