In a large mixing bowl, combine the package of instant pistachio pudding mix with the entire contents of the undrained canned crushed pineapple. Using a whisk, stir the mixture vigorously for about 2-3 minutes until it thickens to a custard-like consistency.

Gently add the thawed Cool Whip to the pudding and pineapple mixture. Using a spatula, fold the Cool Whip in carefully for about 1-2 minutes.

Stir in the mini marshmallows and half of the chopped pecans or walnuts. Use a gentle folding motion for 1-2 minutes.

Transfer the mixture into a serving bowl, cover tightly with plastic wrap, and refrigerate for at least 1 hour.

Remove from the refrigerator, sprinkle remaining chopped pecans or walnuts over the top, and decorate with maraschino cherries.