Preheat your oven to 350F.

Put the gingersnaps in a food processor and process until they are fine crumbs. Add the walnuts and brown sugar, processing again until everything is finely ground. Then add the melted butter and process briefly to combine. Press this mixture firmly into a 9-inch pie plate, covering both the bottom and sides. Bake for 10–12 minutes until golden brown and set aside to cool.

In a saucepan with a heavy bottom, combine sugar, cornstarch, salt, and egg yolks by whisking together until smooth. Gradually whisk in both grapefruit juice and lemon juice. Add a tiny drop of pink gel food coloring.

Heat this mixture over medium heat while whisking or stirring constantly until it comes to a gentle boil. Set your timer for exactly one minute.

Remove from heat and whisk in the butter pieces until melted; the mixture will turn thick and glossy. Strain through a mesh strainer if desired.

Fill your cooled pie shell immediately with the filling. It will start setting fairly quickly as it cools down.

Allow your pie to cool at room temperature before refrigerating uncovered for at least four hours.