In a small saucepan over medium heat, combine the cranberries, sugar, and water. Cook, stirring occasionally, until the cranberries burst and the mixture thickens slightly, about 10 minutes. Remove from heat and let cool. Cut off and discard the rind from the brie cheese. Place the cheese on a parchment-lined plate in the freezer for 20 minutes (this will make it easier to cut). 

Preheat oven to 375℉. Generously grease a 24-cup mini muffin pan with cooking spray and set aside. On a lightly floured surface, roll the puff pastry sheet out into a 10×14-inch rectangle. Cut lengthwise into 4 even strips and then crosswise into 6 even strips, creating 24 squares. Gently press each square into a muffin cup. Prick the bottom with a fork.

Remove chilled brie from the freezer and cut into 24 pieces, approximately 3/4-inch in size.

To assemble, spoon 1 tsp cranberry mixture into each muffin cup, then press in a piece of Brie, top with 1 tsp chopped walnuts, and add a sprinkle of salt. Bake bites in the preheated oven until golden brown, 18 to 20 minutes. Allow to cool slightly, then remove from the pan and serve warm or at room temperature. Enjoy!