For the Crust:

1. Preheat oven to 350°F.
2. Whisk together the spelt flour and salt in a large bowl.
3. Using the paddle attachment on a stand mixer, a food processor, or a pastry cutter, incorporate the butter into the flour and salt until it is broken into pea-sized pieces.
4. Add the ice water and yogurt, and mix until the water is absorbed.
5. Turn onto a clean counter and knead lightly one or two times, to form one smooth disc.
6. Wrap the disc in plastic wrap. Refrigerate for at least 30 minutes to chill the butter.
7. Once the dough has chilled, roll it out on a lightly floured surface until it will fit the base of a tart pan, with a little bit of excess up the sides. Press the dough into the pan, and trim any excess around the edges. Place back in the refrigerator for at least another 30 minutes before filling.

For the Caramelized Onions:

1. In a large skillet, heat the olive oil over medium-low heat. Add the onions and a pinch of salt, and stir them together with the oil.
2. Let this mixture cook for about 20 minutes, stirring occasionally, or until the onions are soft and translucent, and just starting to turn light brown around the edges.
3. Remove from the stove and transfer to a separate bowl or plate.

To Assemble and Bake:

1. Evenly spread the chilled dough with the caramelized onions. Evenly distribute the slices of cheese on top of the onions. Place the whole tomatoes on top, arranging them around the tart.
2. Tear the basil leaves into small pieces and garnish on top. Evenly sprinkle the tart with freshly cracked salt and pepper.
3. Bake for about 40-50 minutes, until crust is golden and tomatoes have slightly shriveled and burst.
4. Remove from oven, and let cool for 15 minutes before serving while still warm.