Preheat oven to 400 degrees F.
Peel, core and thinly slice apples, toss with lemon juice to prevent discoloration,
Roll out thawed puff pastry on a lightly floured surface, 1/8" thick, and cut two 7" rounds.
Transfer pastry rounds to parchment lined baking sheet and arrange apple slices overlapping slightly.
In a small bowl, mix cinnamon and sugar.
Sprinkle with cinnamon sugar. Dot tarts with butter.
Bake in the middle of the at 400 degrees F. for 20-25 minutes, or until golden brown.
Heat apricot jam and water over low heat, or in microwave, stirring until smooth and jam is thinned. Set aside.
Remove tarts from oven, transfer to a wire rack, and brush with apricot jam. Cool.