Farmstead Apple Slab Pie

Spanning the full surface of a baking sheet, slab pies are a great way to feed a hungry crowd. The first time I ate this pie was in Dallas, Texas, at the state fair in the mid-eighties. It was so homey and delicious; the memory of eating it never left me, so I re-created it here, in gluten-free form. After studying baking for the last decade, however, it actually reminds me of a rustic French apple tart, Americanized by replacing the flower-petal apples on top with another layer of tart cru