Preheat oven to 425 degrees F. In a small bowl combine brown sugar, melted butter, and cinnamon. Spread in a 13x9-inch baking pan. Top with apples and/or pears. Roast 15 minutes. Remove fruit. If desired, stir bourbon into sugar mixture in pan.
Line a baking sheet with parchment paper. On a lightly floured surface, unfold puff pastry and cut into nine 3-inch squares. Using a paring knife, make a shallow cut around each square 1/2 inch from the edges. Generously prick centers with a fork. Place 1 inch apart on prepared baking sheet. Bake 10 minutes. Press down centers with back of a spoon.
Spoon apples and/or pears into centers of tarts. Drizzle fruit in each tart with 2 tsp. of the sugar mixture. Sprinkle with walnuts. Bake 7 to 10 minutes more or until pastry is light brown. Drizzle tarts with remaining sugar mixture and, if desired, sprinkle with salt.