Step 1 Heat the oven to 350 degrees. Prick the bottom of the tart shell lightly with a fork, line it with parchment or aluminum foil and fill it with pie weights (you can use dried beans and save them to use as weights the next time you bake). Place the tart shell on a baking sheet and bake until the rim of the tart is dried and set, about 15 minutes (timing may vary depending on the dough or brand). Remove the weights and the aluminum foil and return the tart shell to the oven to bake until it is well browned, about 30 more minutes. Set aside to cool slightly.
Step 2 Meanwhile, prepare the filling. In a small heavy-bottomed saucepan, beat the whole eggs, egg yolks, salt and sugar until smooth and light colored. Add the grapefruit juice, zest, Campari and butter and cook over medium heat, stirring constantly, until the butter melts, about 5 minutes.
Step 3Reduce the heat to medium-low and continue cooking until the curd is thick enough that it coats the back of a spoon and that when you draw your finger across the curd it leaves a definite track, about 5 minutes. The curd should be as thick as thick hollandaise. Remove from heat.
Step 4Pour the curd through a strainer into the pre-baked tart shell and chill until firm. Sprinkle the chopped pistachios over the top before serving.