1. Place walnuts in a single layer on a baking sheet. Bake at 350° for 8 to 10 minutes or until lightly toasted and fragrant. Increase oven temperature to 425°.
2. Unroll piecrust on a flat surface. Cut into 24 rounds using a 2-inch round cutter. Press rounds into bottoms of ungreased miniature muffin cups (dough will come slightly up sides, forming a cup). Prick bottom of dough with a fork.
3. Bake at 425° for 6 to 7 minutes or until golden. Remove from pans, and cool on a wire rack 15 minutes. Reduce oven temperature to 300°.
4. Meanwhile, cut Brie round into 24 pieces. Stir together grapes and next 4 ingredients.
5. Arrange pastry shells on a baking sheet. Place 1 Brie piece in each pastry cup; sprinkle Brie pieces evenly with toasted walnuts.
6. Bake at 300° for 4 to 6 minutes or just until cheese begins to melt. Top tarts evenly with grape mixture. Garnish, if desired. Serve immediately.