Heat sugar in a small saucepan over medium heat. Stir constantly as the sugar melts, about 5-6 minutes, until it turns into a golden liquid.
Whisk in butter until thoroughly combined, then whisk in heavy cream until incorporated. Let bubble for about one minute, then remove from the heat. Stir in the sea salt and ground cardamom. Allow to cool.
Meanwhile, preheat oven to 350°F.
In a mixing bowl, combine all-purpose flour, powdered sugar, and salt. Add melted butter and stir until a dough forms.
Pour the dough onto the round blue steel baking sheet with the high-sided fluted perforated stainless steel tart ring, pressing the dough evenly along the sides and bottom.
Poke the dough all over with the tines of a fork then transfer to the oven.
Bake for 20 minutes, until light golden brown. Allow the tart crust to cool as you prepare the apples.
Remove the core from the apples by slicing around the core four times. Slice the apple pieces into very thin slices, about ⅛ inch thick.
Pour the prepared caramel in a thin even layer along the bottom of the tart crust using a spoon or offset spatula to spread.
Arrange apple slices vertically along the edge of the tart crust, overlapping halfway over each slice. Continue layering until you reach the middle. The apple slices may start to break as you get closer to the center. You may need to slice the apples a little thinner, if necessary.
For the final center, take a few apple slices and line them up overlapping each slice. Roll the apples up to form a small rose then place them in the center of the tart.
In a small bowl, combine sugar and ground cardamon. Sprinkle evenly across the apples.
Cut the unsalted butter into small pats and disperse evenly across the apples and sugar.
Bake for 45-50 minutes, until the crust is dark golden brown and the sugars start bubbling.
Allow to cool, then dust with powdered sugar.
