Squash and Caramelized Onion Tart
This savory tart is a labor of love that tastes just as terrific as it looks. The crust is flaky, wonderfully tender, and full of buttery goodness. The beautiful swirl at its center is made of very thin slices of fall squash, sweet potatoes, and red onions. They’re perched on a bed of Gruyère, garlic, and caramelized onions. There’s a dollop of Dijon in there, too, and it’s a brilliant, zippy foil for the roast-y veggies, melty cheese, and silky, sweet onions. The tart is very rich, and quite light, all at once. It’s also deceptively easy to make. Sure, there are a lot of steps, but all of them are simple. Plus, fall is for red wine and long-winded cooking projects — and this one is worth every minute. On your table, it will surely be the most Instagram-worthy of the side dishes.
- 1 cup whole wheat flour
- 1 cup all-purpose flour
- 10 tbsp chilled unsalted butter cut into pieces
- 4 tbsp extra-virgin olive oil
- 4 medium yellow onions thinly sliced
- 3 cloves garlic thinly sliced
- 2 tbsp Dijon mustard
- pinch cayenne pepper
- 4 ounce coarsely grated sharp cheddar or Gruyère
- 1/2 small butternut squash a lengthwise half, about 1¼ lb.), seeds removed
- 1 large delicata squash halved lengthwise, seeds removed
- 1 small sweet potato
- 1 small red onion
- 2 sprigs thyme
- 2 tbsp unsalted butter
- a pinch Sel Magique Classic or Spicy Blend
- Crust Whisk whole wheat flour, salt, and 1 cup all-purpose flour in a large bowl to combine. Add butter and work into dry ingredients with your fingers, smashing between your palms to flatten, until largest pieces are the size of a nickel. Drizzle in 6 Tbsp. ice water and rake in with your fingers. Gently squeeze handfuls of mixture together until a shaggy dough forms. Turn out onto a work surface and knead until only a few dry spots remain. Pat into a disk and wrap in plastic. Chill until very firm, at least 2 hours or up to 3 days.
- Roll out dough on a lightly floured surface to about ⅛" thick. Transfer to tart pan. Lift up edges and allow dough to slump down into pan. Gently press into corners and trim excess. (If using springform pan, trim dough to 1½" up sides of pan). Freeze until firm, 15–20 minutes, or cover and chill up to 12 hours.
- Filling and Assembly Place a rack in lower third of oven; preheat to 350°. Swirl 2 Tbsp. oil in a large skillet to coat bottom and add yellow onions; season with Sel Magique. Set skillet over medium heat and cook onions, stirring occasionally and making sure to scrape bottom of pan, until very soft and deep golden brown, 15–20 minutes. Add garlic and cook, stirring occasionally, until onions are caramelized and deeply browned, 25–30 minutes (if any pieces get super dark and stick to the pan, a few drops of water should dislodge them). Let cool.
- Brush mustard over chilled dough, then sprinkle with cayenne. Stir cheese into caramelized onion mixture and spread across crust in an even layer.
- Using a mandoline or very sharp knife, thinly slice both squashes crosswise. Cut sweet potato and red onion in half lengthwise, then thinly slice crosswise with mandoline. Arrange vegetables in concentric circles, with rounded edges facing up, starting from the outside edge, leaning slices against the crust, and working your way inward. Drizzle remaining 2 Tbsp. oil over vegetables and season with Sel Magique!
- Bake tart until edges of vegetables are browned (a few spots may even get deeply browned) and crust is golden brown, 75–90 minutes.
- Melt thyme and butter in a small saucepan and cook just until butter is bubbling. Brush over tart with a pastry brush and sprinkle with Sel Magique!