Savory Apple Tart

Heat oven to 400 degrees F and get out a rimmed 10 x 15 baking sheet.

For crust, whisk together the flours and 1 teaspoon of salt in a large bowl.Cut 2 sticks of chilled butter into cubes. Work the butter into the mixture quickly with your fingers until the mixture is crumbly, but small chunks of butter remain. When the butter and flour are combined, toss the mixture gently with a fork while drizzling in ice water. When all the water is incorporated, begin mixing and gently squeezing with your hands until dough is formed. Scrape onto a floured counter and shape into a rectangle. Let the dough rest for 30 minutes.

Roll out the rested dough until it is an inch longer and wider than the pan. Gently fold the dough in half and place on the pan, then unfold. Pat the dough into the pan and fold under any extra dough at the rim. Cover with foil and bake for 20 minutes.

For filling, melt 2 tablespoons of butter in a large saucepan over medium heat. Add onions and thyme and cook, stirring frequently, until the bottom of the pan is coated with brown reduction, about five minutes. Add 2 tablespoons of water and scrape up reduction with a wooden spoon. Repeat cooking, adding water and scraping up browned bits until onions are softened completely and have a deep brown color, about 15 minutes. Add ½ teaspoon of salt and pepper. Remove from heat and let cool.

To assemble the tart, spread onions on the prepared crust. Add a thin layer of Dijon mustard, then cover with sliced apples. Sprinkle shredded cheddar on top and bake for 25 minutes, until apples are tender and cheese is melted and golden. Place on a rack to cool. Serve warm or let cool and refrigerate, tightly covered, for up to 3 days.