Apple, Potato and Onion Gratin

This is a fabulous dish! We absolutely love it, although we will mark it as "special occasion" given the amount of butter -- it would certainly have Paula Dean's blessing. I cut it in 1/2 from the original (provided by bon apetit) and the directions were difficult to follow so I've reworked them below.

Ingredients:

• 6 tablespoons, divided
• 1 large sweet/white onion, sliced
• 2 tablespoons (packed) chopped fresh thyme
• 2 teaspoons fine sea salt, divided
• 1/3 cup water
• 1/3 cup dry white wine
• 2 teaspoons sugar
• 1 1/4 pounds Yukon Gold, yellow Finn, or German Butterball potatoes, peeled, cut into 1/4-inch-thick rounds
• 1 pound tart apples (such as Granny Smith, Pippin, or Pink Lady), peeled, halved, cored, cut into 1/4-inch-thick slices

Preparation

1. Preheat oven to 400°F.
   
2. Butter 9x6-inch glass or ceramic baking dish.
   
3. Melt 3 tablespoons butter in large nonstick skillet over medium heat. Add onions, thyme, and 2 teaspoons salt; sauté until onions are translucent, about 10 minutes.
   
4. Increase heat to medium-high; sauté until onions are tender and begin to color, about 8 minutes longer.
   
5. Add remaining 3 tablespoons butter, 1/3 cup water, wine, and sugar to skillet; stir and swirl skillet to combine. Bring to boil.
   
6. Remove from heat and cool onion mixture to lukewarm.
7. Combine potatoes, apples, remaining 2 teaspoons salt, and onion mixture in large bowl; toss gently to blend.
   
8. Transfer to prepared baking dish, spreading evenly.
   
9. Cover dish with parchment paper, then cover with foil, shiny side down. Bake gratin until potatoes are tender, about 55 minutes.
   
10. Uncover and bake until top browns and juices bubble thickly, about 20 minutes longer.
11. Let gratin stand 15 minutes before serving.