Preheat your oven to Steam, 212°F/100°C (100% humidity). Grease 6 cups of a muffin pan really well, and have a shallow, lightweight stainless steel pan ready for the apple slices.
Cut the pastry into six rectangles (three from each sheet), each measuring roughly 3x10"/8x25cm. Use a very sharp knife and try to cut straight down through the pastry, rather than dragging the knife through it. Cleaner cuts make for puffier layers later on. Return the pastry to the fridge while you prepare the apples.
2 sheets puff pastry
Cut each apple in half through the core. Remove the core, then slice the halves very thinly using a sharp knife or a mandolin. You want the slices to be not quite paper thin, but almost. As you cut, keep the slices together. Get your oven pan, and spread out the slices in rows, overlapping each slice, until you have 6 rows of overlapped apple, each 10"/25cm long. You'll likely have some apple left over, so use the 'best' slices cut from the larger parts of each apple for your rows, and snack on the rest while you work.
2 apples
Put the apples into the preheated oven for 5 minutes. No longer! You're aiming just to par-cook it long enough that it's pliable and able to be rolled up without breaking. Remove the pan from the oven and change your oven settings to Combi Steam, 375°F/190°C, 50% (med) humidity.
While the apples cool down, mix the cinnamon and granulated sugar in a small bowl. Remove the pastry from the fridge and sprinkle cinnamon sugar over half of each rectangle, running longways.
1 tsp cinnamon, 2 Tbsp granulated sugar
Carefully lift each row of cooled apple slices and lay it on top of the cinnamon sugar pastry. The lower half of each pastry rectangle will be uncovered, with the rounded (skin side) edges of the apples lying above the top edge of the pastry. See photos for examples. Fold the other half of the pastry up over the apples, so you have a long, skinny double layer of pastry, with the apples and sugar inside it.
Roll up each pastry rose, so you've got little pinwheels of pastry-apple-pastry. Press the edge of the pastry down when you get to the end, to make a neat finish, then put each apple rose, fruit side up, into your greased muffin pan.
Bake the apple roses until the pastry is puffed and golden brown, about 25-30 minutes. The tops of the apples will start to brown in the last few minutes of cooking, but don't be tempted to pull them too early as the pastry in the centers takes a while to cook through.
Dust the hot apple roses with powdered sugar and serve barely warm or at room temperature.
2 Tbsp powdered sugar