Preheat the oven to 375F
In a large bowl add the graham cracker crumbs and sugar and stir to combine. Add the maple syrup and the 5 tablespoons of melted butter and stir to thoroughly combine.
Grease generously a 10-inch cast-iron skillet with a tablespoon of room-temperature butter. Pour the dough into the skillet and lightly press it into shape. Line with aluminum foil and fill with pastry weights or uncooked rice. Bake for 10 minutes.
Allow cooling on a wire rack before filling.
Preheat the oven to 325F.
In a medium bowl, combine the condensed milk, lemon juice, egg yolks, and vanilla.
Working with the crust in a 10-inch cast-iron skillet, pour the filling into the crust. Top with very thin slices of lemon. Arrange them as you wish.
Put the skillet in the oven and bake for about 15 to 20 minutes, until the liquid has set into a soft custard.
Remove the skillet from the oven and allow to cool completely before serving.