In a medium-sized bowl, whisk together the instant pistachio pudding mix and milk. Follow the instructions on the pudding mix package (usually about 2 minutes of whisking until it thickens).
Once thickened, fold in the thawed whipped topping until the mixture is smooth and creamy.
Assemble the Pie:
Pour the pistachio mixture into the prepared graham cracker crust. Use a spatula to spread it evenly.
Chill:
Refrigerate the pie for at least 2-3 hours, or until set. For best results, chill it overnight so the flavors can meld and the pie can firm up.
Garnish and Serve:
Before serving, garnish the pie with crushed pistachios and extra whipped topping, if desired.
