Preparation

做面团：

• 倒入面粉,糖,盐在一个大碗里. Add half of the butter and gently toss with your hands to coat each piece with flour. Using a pastry cutter or two table knives, cut the butter into the flour until it’s the texture of coarse meal. Add the remaining butter, gently toss again to coat each piece, and quickly 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 1/2 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.)

Make the topping:

• 用中火加热12英寸的煎锅1分钟，然后加入橄榄油和黄油。黄油融化后，加入洋葱，百里香和1/2茶匙。盐和炒，直到洋葱变软并浅褐色，为6至8分钟。品尝洋葱并在必要时添加更多盐 - 它们应该经过良好调味。将洋葱转移到一个中等大小的碗中，放在一边冷却。（这可以直到未来一天；保持覆盖和冷藏。）

Roll the dough:

• 从冰箱中取出一个矩形。将其切成两半。一次与一半一起工作，将其滚动在略微面粉的表面上，成1/8英寸厚的狭窄矩形，长约14英寸，宽5英寸。（如果面团太冷以至于无法轻易滚动，请在室温下在衬有羊皮纸的烤盘上加热（两个狭窄的矩形应并排配合），上面放上第二张羊皮纸，然后冷藏。dough and refrigerate until you’re ready to prepare the tarts. (This may be done up to 4 hours ahead; keep covered. To conserve space, stack all four rectangles on one baking sheet between layers of parchment.)

Assemble the tarts:

• 食用前约1小时，将烤箱架放在烤箱的上和下三分之一中，然后将烤箱加热至375°F。食用前约40分钟，将洋葱分成四个相等的部分，并将其散布在面团矩形上的薄薄的层中，留下1英寸的边框。(You may not need to use all the onions; a thick layer will weigh down the tart slices and they’ll flop when your guests pick them up.) Fold the exposed border over the onions—if the dough is too thick in the corners, trim as necessary. Brush the folded edges with the egg. Bake the tarts until the crust is well browned, about 30 minutes, switching the positions of the baking sheets halfway through. Remove the tarts from the oven and let cool briefly on the baking sheet. Cut each tart into eight slices and serve.