To Make the Crust:

1. Add buckwheat flour, all-purpose flour, and salt to a food processor. Pulse to combine. Add cold cubed butter and thyme. Pulse again until the mixture forms pea-sized crumbles.
2. Add the apple cider vinegar, and pulse a few times to combine. Add cold water one tablespoon at a time, pulsing once or twice between each addition, until dough holds together but isn’t wet.
3. Remove dough from food processor and form into a flattened disk. Wrap in plastic wrap and refrigerate for at least 30 minutes, or up to overnight.
4. Preheat oven to 400°F and lightly grease an 11-inch-round fluted tart pan by running a stick of cold butter over the bottom and sides.
5. While oven preheats, lightly flour a clean work surface and roll out dough into a ¼-inch-thick circle about 13 inches in diameter.
6. Transfer dough to prepared tart pan and press into the bottom and up the sides, trimming off any excess dough to make the sides even. Next, prick holes all over the bottom of the dough with a fork.
7. Lay a piece of parchment paper over the top of the dough, making sure that the paper is large enough to create an overhang going up and over the sides of the tart pan so that it’ll be easy to remove after baking. Fill with pie weights.
8. Place crust in oven and blind bake for 15 minutes. Remove weights and parchment paper, then bake another 10-12 minutes, or until edges are lightly browned and the top looks nearly dry. Remove from oven and allow to cool. Keep the oven on.

To Make the Filling:

1. If your coconut oil is solid at room temperature, melt ½ tablespoon in the microwave. In a large mixing bowl, toss cubed sweet potatoes with ½ tablespoon melted coconut oil and ½ teaspoon each of salt and nutmeg.
2. Spread mixture in an even layer on a large baking sheet and bake for 20-25 minutes, or until sweet potato cubes are soft and starting to brown. Remove from oven and set pan aside to cool until ready to use.
3. While sweet potatoes are roasting, place a large skillet over medium heat. Add 1 tablespoon coconut oil and minced garlic. Cook until garlic is fragrant, about 1 minute, stirring frequently.
4. Add chopped Swiss chard, red pepper flakes, and ¼ teaspoon salt. Saute until chard is wilted and reduced in size, about 5 minutes. Transfer to a large mixing bowl and set aside.
5. Using the same pan that you sauteed the chard in, place over medium heat and add remaining ½ tablespoon coconut oil. Once oil is hot, add sliced onion and ¼ teaspoon salt. Stir every few minutes until onion has caramelized, about 20 minutes. Once onions are golden brown, add balsamic vinegar, stir, and remove pan from heat. Set aside.
6. Using a paper towel, squeeze out excess water from the Swiss chard, then return it to the bowl. Add about ¾ of the roasted sweet potatoes, half of the Pecorino cheese, the caramelized onions, and ¼ teaspoon black pepper.
7. Whisk eggs in a small bowl, and add to the vegetable mixture. Stir to combine well.
8. Spread filling in baked tart crust and then sprinkle remaining sweet potatoes and cheese over the top.
9. Bake for 25-30 minutes, or until set and the top is lightly browned. Remove from oven and cool for 10 minutes before removing from the pan and slicing.