Onion Tart

Combine the flour, sugar, and salt in a large bowl. Add half of the butter. Toss with your hands to coat each piece with flour. Using a pastry cutter, cut the butter into the flour until it is the texture of coarse meal. Add the remaining butter, gently toss again to coat each piece, and cut again until the larger pieces are about the size of large peas. In two or three additions, sprinkle about 1/3 cup ice water into the bowl, lightly tossing the mixture between your fingers to moisten it evenly. Stop adding water when the dough looks ragged and rough but holds together when you gently squeeze a small clump in your palm. Shape the dough into a brick (be careful not to knead it, just squeeze it gently into a solid mass). Cut the dough in half crosswise. Press each half into a flat rectangle about ½ inch thick and wrap tightly in plastic. Refrigerate the dough for at least 2 hours. (The dough can also be refrigerated for up to two days or frozen for up to one month. Thaw it overnight in the refrigerator before using.)

Heat a skillet over medium-high heat for 1 minute and add the olive oil and butter. Add the onions, thyme, and ½ t salt and sauté until the onions are tender and lightly browned, 6 to 8 minutes. Transfer the onions to a medium bowl and set aside to cool. (This may be done up to a day ahead)

Cut one rectangle in half crosswise. Working with one half at a time, roll it on a lightly floured surface into a 1/8-inch-thick narrow rectangle, about 14 inches long and 5 inches wide. (If the dough is too cold to roll easily, let it warm at room temperature on a baking sheet lined with parchment (two narrow rectangles should fit side by side), top with a second sheet of parchment, and refrigerate. Repeat with the remaining dough and refrigerate until you’re ready to prepare the tarts. (This may be done up to 4 hours ahead; keep covered.)