12
tablespoons butter - (1 1/2 sticks) divided
2
pounds onions sliced
2
tablespoons chopped fresh thyme - (packed)
4
teaspoons fine sea salt divided
2/3
cup water
2/3
cup dry white wine
4
teaspoons sugar
2 1/2
pounds Yukon Gold, yellow Finn, or German Butterball potatoes peeled, and cut into 1/4"-thick rounds
2
pounds Granny Smith, Pippin, or Pink Lady apples peeled, halved, cored, cut into 1/4"-thick slices
Preheat oven to 400 degrees. Butter 13- by 9- by 2-inch glass or ceramic baking dish. Melt 6 tablespoons butter in large nonstick skillet over medium heat. Add onions, thyme, and 2 teaspoons salt; sauté until onions are translucent, about 10 minutes. Increase heat to medium-high; sauté until onions are tender and begin to color, about 8 minutes longer. Remove from heat. Add remaining 6 tablespoons butter, 2/3 cup water, wine, and sugar to skillet; stir and swirl skillet to combine. Bring to boil. Cool onion mixture to lukewarm. Combine potatoes, apples, remaining 2 teaspoons salt, and onion mixture in large bowl; toss gently to blend. Transfer to prepared baking dish, spreading evenly. Cover dish with parchment paper, then cover with foil, shiny-side down. Bake gratin until potatoes are tender, about 55 minutes. Uncover and bake until top browns and juices bubble thickly, about 20 minutes longer. (Can be made 6 hours ahead. Let stand uncovered at room temperature. Rewarm, loosely covered with foil, in 300 degrees oven for 20 minutes.) Let gratin stand 15 minutes before serving. This recipe yields 8 servings.
