Potato and Apple Gratin

I recently ordered the book Spice Companion by Lior Lev Sercarz on Amazon and then discovered the New York City store called La Boîte.
I was looking for a Fall recipe with apples and came across this one. I am going to make it again for Thanksgiving because it was so delicious.

Ingredients

2 1/2    pounds baking potatoes (eg. Yukon Gold, Russet, and/or Kennebec),
peeled and sliced 1/4″ thin
2           pounds apples with good acidity (eg. Honeycrisp, Winesap, and/or Pink Lady)
peeled and cored, sliced 1/4″ thin
8           tablespoons (1 stick) butter, diced
2           medium onions, sliced into thin rounds
1            teaspoon salt
1/2       cup dry white wine
1/2       cup chicken or vegetable stock*

2 1/2    teaspoons Lula N.41 blend

1 parchment paper circle cut to fit the skillet, or a lid that is slightly smaller than the pan

* for additional body, add a teaspoon of gelatin to the stock

Instructions

Preheat oven to 400°F. Butter a 10 inch cast iron skillet or baking dish and add a layer of half of the potatoes.

In another skillet melt 6 tablespoons of the butter and sauté the onions with the salt over medium heat until golden and fragrant (10-15 minutes). Add the wine, stock, and the Lula blend and cook until the liquid is reduced and mostly absorbed.

Place the cooked onions in a layer over the potatoes, followed by a layer of apples then the remaining potatoes.

Cover with the parchment circle or lid and and bake for 40 minutes.

Uncover, dot with the remaining butter and bake (10-15 minutes) or broil (timing will vary) until the top is crisp.