Tomato Galette with Gruyere and Caramelized Onione

Do Ahead

1. Make the pie dough. Wrap in plastic wrap and refrigerate for at least 30 minutes or for up to 24 hours. If frozen, allow the dough to thaw completely.

Make the Galette

1. In a large skillet, heat the oil over medium-high heat. Add the onions and cook until browned all over, stirring frequently. About 30 to 40 minutes. Set aside.
2. Preheat oven to 400°F.
3. On a lightly floured surface roll out the dough to a circular shape about 10 to 12 inches in diameter. Add more flour to the surface as needed to prevent sticking.
4. Gently transfer the dough to a pizza stone or baking sheet. (Before doing this, you can sprinkle the stone or baking sheet with semolina flour or fine cornmeal. This isn't completely necessary, but helps to ensure the dough won't stick to the stone).
5. Add the caramelized onions in the center of the dough, leaving about 1 to 2 inches of dough around the edges. Place the cheese over the onions, and then arrange the tomato slices over the cheese (you may not use all of the tomato slices).
6. Liberally sprinkle salt and pepper over all of the tomatoes and then sprinkle on the chopped thyme.
7. Use your hands to gently pull the edges of the dough toward the center of the galette. Bring the sides of the dough so about ⅔ of the tomatoes are visible.
8. In a small bowl, mix the lightly beaten egg with 1 tablespoon of water. Brush the egg wash all over the top portion of the dough.
9. Bake for 35 to 40 minutes, or until the dough is lightly browned and the interior is bubbly. Use a couple of flat spatulas to gently transfer the galette from the stone (or baking sheet) to a cutting board. Allow to rest for 15 minutes before slicing. Serve warm or at room temperature.