Preheat the oven to 400° F.
Unfold one puff pastry sheet and cut off the corners to create a circle that fits nicely into into the bottom and up the sides of a tart pan. Use the puff pastry corners to fill in any areas that need little extra crust.
Peel, core and slice the apples thinly and place them in a bowl. Drizzle lemon juice over top to keep the apples from browning.
In a separate bowl, mix together the oats, brown sugar, flour, cinnamon and melted butter to create a crumbly topping.
Arrange the apple slices on in the tart pan in an alternating pattern. Top the tart with the crumble and bake for 15-20 minutes on a low oven rack until the pastry sheet is golden brown.
Cool and serve as is or with a scoop A vanilla ice cream.