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1
Heat the oven to 375 F. Sprinkle the lemon juice evenly onto the apples to prevent them from discoloring.
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2
Place sugar in a cast-iron skillet and melt over medium heat.
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3
Uncover and cook, stirring occasionally until the sugar is a light butterscotch color and its temperature reaches 320 degrees (measure with a candy thermometer).
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4
Turn off the heat.
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5
The temperature will continue to rise.
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6
Once the temperature reaches 350 degrees, whisk in the butter, 1 piece at a time.
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7
Make sure each piece is incorporated before adding the next, then add the salt.
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8
Cover the caramel with the apple slices, starting in the center and overlapping in a spiraling.
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9
Turn the heat back on and cook for 15 minutes.
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10
Stir to make sure the apples are cooking evenly, but be sure to return the pieces to the original configuration.
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11
Cover the pan with the puff pastry.
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12
Tuck in the edges around the apples and prick the pastry with a fork.
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13
Bake until the edges are deep amber in color and the pastry is puffed and golden, 40 to 45 minutes.
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14
Remove from the oven and cool for 10 minutes before inverting onto a plate larger than the pan.
