LèMert Christmas Recipes – Squash and Caramelized Onion Tart

We’re back with another delicious Christmas recipe, today we have a healthy Squash and Caramelized Onion Tart that’s packed with flavor! Smooth squash and sweet onions pair perfectly with the sharp cheese, mustard and the soft buttery crust. This recipe can be great as part of a light festive brunch, don’t forget the mimosas!



130g Whole wheat flour

1 Tsp kosher salt

130g All-purpose flour, plus more for surface

145g Tbsp. chilled unsalted butter, cut into pieces


4 Tbsp. extra-virgin olive oil, divided

4 Medium yellow onions, thinly sliced

Kosher salt

3 Garlic cloves, thinly sliced

2 Tbsp. Dijon mustard

Pinch of cayenne pepper

120g Coarsely grated sharp cheddar or Gruyère

½ Small butternut squash a lengthwise half, seeds removed

1 Delicata squash, halved lengthwise, seeds removed

1 Small sweet potato

1 Small red onion

2 Sprigs thyme

2 Tbsp. unsalted butter

Flaky sea salt


An 11"- or 12"-diameter fluted tart pan with removable bottom or a 9"-diameter springform pan



Mix whole wheat flour, salt, and 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. 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.


Place a rack in lower third of oven; preheat to 180 Celsius. Swirl 2 Tbsp. oil in a large skillet to coat bottom and add yellow onions; season with kosher salt. 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 mandolin or very sharp knife, thinly slice both squashes crosswise. Cut sweet potato and red onion in half lengthwise, then thinly slice crosswise with mandolin. 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 kosher salt.

Bake tart until edges of vegetables are browned don’t worry if a few spots 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 sea salt.