Preheat the oven to 375 degrees F. Line a baking sheet with parchment paper.
Unroll one of the pie crusts onto a cutting board. Using a 2 1/2-inch plain-edged round cutter, cut out 8 circles from the crust. Place the circles on the parchment-lined baking sheet. Discard the remaining dough.
Unroll the second pie crust onto the cutting board. Using the same 2 1/2-inch round cutter, cut out 8 more circles. Then, use a 2-inch plain-edged round cutter and cut out 8 smaller circles from the larger circles on the board. Set aside the rings. Using a 1 1/2-inch Christmas tree cutter, cut out 8 small trees from the smaller full circles. You should now have eight 2 1/2-inch full circles, eight 2 1/2-inch rings and eight small Christmas trees of dough.
Beat the egg with 1 tablespoon of water. Use your finger or a brush to brush the edge of the 2 1/2-inch circles. This will help the rings adhere during baking. Lay the rings on top of the egg-washed circles. Working carefully, spoon 3 cherries into the center of each tart. Place a tiny tree on top of each tart over the cherries. The trunk of the tree should be touching the outer ring. Using your finger or a pastry brush, brush the pie-crust ring and tree with the egg wash. Bake the tarts until the dough is golden brown, about 15 minutes. Allow to cool completely.
Create a bed of rosemary on a platter or cake stand and arrange the baked tarts on top of the rosemary. Dust with confectioners' sugar and enjoy!