Pastry 1/3 cup old fashioned oats
2 tablespoons sugar
1 1/4 cups flour
1/8 teaspoon salt
1/2 cup unsalted butter, chilled — cut in 1/2″ pieces
4 tablespoons ice water
Filling 6 tablespoons unsalted butter
1/3 cup sugar
1/3 cup brown sugar — packed
1 3/4 teaspoons lemon peel — grated
2 1/2 pounds Golden delicious apples, peeled, halved — cored
each apple cut into 3 wedges Unsweetened whipped cream — garnish, optional
For pastry: Blend oats and sugar in a processor until oats are finely ground. Blend in flour and salt. Add butter and process using the on/off button, until mixture resembles coarse meal. Mix in ice water by the tablespoon, until mixture begins to clump together. Gather dough into ball, flatten into disk. Wrap in plastic; chill 1 hour. (Can be made 2 days ahead. Keep chilled. Soften at room temperature before rolling.)
For filling: Preheat oven to 425. Melt butter in a heavy 10″ diameter cast iron skillet over medium heat. Gradually mix in both sugars; stir until sugars begin to dissolve and mixture starts to bubble, about 2 minutes. Mix in lemon peel. Remove from heat. Arrange apple wedges on their sides in a circle around the eges of the skillet, so they fit tightly. Place several apple wedges tightly together in center of skillet. Cut any remaining apple wedges into 1/2″ pieces; scatter over apple wedges, mounding slightly in center.
Place skillet over medium heat and cook 3 minutes. Cover skillet and cook apples 5 minutes. Uncover skillet and cook until apple juices and sugars form syrup but apples are still firm to touch, about 10 minutes. Remove from heat.
Roll out dough between sheets of floured wax paper to 12″ round. Peel off top sheet of paper. Using bottom sheet of waxed paper as an aid, invert dough onto warm apples in skillet. Remove wax papper. Press pastry edges down and inside skillet around apples.
Bake until crust is deep golden, about 40 minutes. Using a small knife, cut around crust to loosen. Let stand 1 minute. Place a large platter over skillet. Using oven mitts as an aid, hold platter and skillet tightly together and invert tart onto platter. Rearrange any apples that may have become dislodged. Cool to lukewarm, about 30 minutes. Cut into wedges. Garnish with a dollop of unsweetened whipped cream on top of slice.
Note: As the tart cools, the crust gets flakier, so it’s best not to have this right out of the oven. Try it lukewarm or at room temperature.