Directions

For the crust:

Add flours and the salt in a food processor; pulse to combine. Add the oil and butter and pulse until it breaks down to the size of small peas.

Sprinkle 4 tablespoons of the ice water over the mixture and pulse again until the pastry just holds together (if it’s too dry to hold together, pulse in tiny amounts of the water until it holds).

Shape it into a thick disk. Wrap the dough in the plastic wrap and refrigerate for at least half an hour.

Meanwhile, position a rack in the center of the oven and preheat the oven to 375°F.

Unwrap and roll the dough on a lightly floured working surface into a 14-inch circle. (If the pastry cracks, just press it back together.)

Wrap the pastry around the rolling pin and unroll it over an 11-inch fluted tart pan with a removable bottom.

Without stretching the dough, very gently work it into the pan, pressing the pastry against the sides.

Roll the pin over the pan to trim the excess dough.

Use the scraps as needed to make the edge even and about ¼ inch thick (at the narrowest points).

Press gently all around the edge so the dough comes up slightly above the rim of the pan.

Prick the bottom of the crust all over with a fork, line with parchment, and fill with dried beans or pie weights.

Put the tart pan on a baking sheet and bake until the edge looks dry, about 10 minutes.

Carefully remove the beans and parchment and bake until the bottom is just set and looks dry, 5 to 7 minutes more.

Cool on the baking sheet on a wire rack.

For the filling:

Scrub the sweet potatoes, poke them once or twice with a fork, and put them on a small foil-lined baking sheet.

Roast until tender when pierced, 50 to 60 minutes. Cool, peel, and cut into ¾ inch dice.

While the sweet potatoes are roasting, heat 1 tablespoon of the olive oil in a large nonstick skillet over medium-high heat.

Heat 1 tablespoon olive oil in the pan over medium-low heat. Add the garlic and cook until fragrant and just beginning to color, about 2 minutes.

Increase the heat to medium high, stir in the chopped kale or chard and a pinch of salt, and toss over medium-high heat until the chard is completely wilted, 4 to 5 minutes.

Drizzle the remaining 1 tablespoon of balsamic vinegar over the chard and toss it until the vinegar evaporates, 1 to 2 minutes. Transfer to a large bowl.

Stir in the diced sweet potatoes, ⅔ of the cheese, and cayenne pepper to the chard; mix until well combined.

In a small bowl, add the eggs, a pinch of salt and pepper, whisk until well blended and pour into the kale mixture.

For baking the tart:

Spread the filling evenly in the tart shell and sprinkle the remaining cheese on top. Bake the tart on a baking sheet until the cheese is nicely browned, 26 to 32 minutes.

While the tart is baking, heat the remaining tablespoon of oil in the pan over medium-high heat.

Stir in the onions and a generous pinch of salt, cook, stirring, until the onions wilt and develop dark-brown charred spots, 10 to 12 minutes.

Reduce the heat to medium low, cover the pan, and stir frequently, until softened and caramelized, about 9 minutes more.

Stir in 1 tablespoon of balsamic vinegar until it evaporates and glazes the onions. Remove from the heat and set aside.

When the tart is baked, arrange the balsamic onions over the top of the tart, allowing bits of cheese to peek through.

Allow the tart to cool for 8 to 10 minutes. Remove the rim of the pan. Slice and serve warm..