Prep The Pie Dough (if you're using)

1. Make sure your butter and buttermilk are super cold. Process the flour, baking powder, black pepper, Herbs de Provence, and salt in a food processor until combined.

2. Add 1/4 cup of gruyere cheese to the flour mixture and pulse 5 times.

3. Add the cold butter, coat butter with flour mixture, and spread out into an even layer. This helps the butter process more evenly. Pulse until butter breaks down into pea-sized pieces, about 7 pulses. Some pieces will be smaller or larger, that’s fine. Don’t over-pulse it.

4. Turn out onto a lightly floured surface and gently knead/press into 1 large dough ball. Refrigerate for at least 30 minutes.

Compile The Tart

1. In a cast-iron pan, sauté butter and onion, stirring occasionally and scraping bottom of pan, until onions are soft and deep golden brown, 15-20 minutes. Add garlic and cook for another 10 mins (if any pieces get super dark and stick to the pan, a few drops of water should dislodge them). Let cool.

2. Let dough come to room temperature for 10 mins, so it’s easy to roll. Lightly flour your surface and rolling pin and roll out into desired thickness (1/4” inch thick is good). Place in pie pans and trim off excess.

3. Brush mustard over chilled dough, then sprinkle with cayenne. Stir Gruyere cheese into onion mixture and spread across crust in an even layer.

4. Cut squash, sweet potato, and red onion in half lengthwise, then thinly slice crosswise with mandoline. Arrange in concentric circles, with rounded edges facing up, starting from the outside edge, leaning slices against the crust, and working your way inward. Drizzle 2 tbsp olive oil over each pan of vegetables and season with kosher salt.

5. Bake until edges of vegetables are browned (a few spots may even get deeply browned) and crust is golden brown, 75–90 minutes.

6. 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. Let cool before serving - ENJOY!