Apple, Potato And Onion Gratin - cooking recipe

Ingredients

12 tablespoons butter, divided
2 lbs onions, sliced
2 tablespoons packed chopped fresh thyme
4 teaspoons fine sea salt, divided
2/3 cup water
2/3 cup dry white wine
4 teaspoons sugar
2 1/2 lbs yukon gold potatoes or 2 1/2 lbs similar potatoes, peeled, cut into 1/4 inch thick rounds
2 lbs tart apples (such as granny smith)

Preparation

preheat oven to 400 degrees. butter a 13x9 pyrex.
melt 6 T butter in a large nonstick skillet over medium heat. add onion, thyme and 2 t salt; saute until onion are translucent, about 10 minutes.
increase heat to medium-high; saute until onion are tender and begin to color, about 8 minutes longer. remove from heat. add remaining 6 T butter, 2/3 c water, wine, and sugar to skillet; stir and swirl skillet to combine. bring to a boil, then cool onion mixture to lukewarm.
combine potatoes, apples, remaining 2 t 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 degree oven for 20 minutes.) let gratin stand 15 minutes before serving.