Goat Cheese and Onion Tarts

PreparationMake dough: Cut butter into bits and in a bowl with a pastry blender or in a food processor blend or pulse together with flour and salt until mixture resembles coarse meal. Add 2 tablespoons ice water and toss with a fork or pulse until incorporated. Add enough remaining ice water, 1 tablespoon at a time, tossing with fork or pulsing to incorporate, to form a dough. On a work surface smear dough with heel of hand in 3 or 4 forward motions to make dough easier to work with. Divide dough in half and pat each half into a rectangle, about 6 by 3 inches. Chill rectangles, wrapped separately in plastic wrap, at least 1 hour and up to 1 week. Thinly slice onions and in a large skillet cook with salt to taste in oil, covered, over moderate heat, stirring occasionally, 10 minutes, or until softened. Cook onions, uncovered, stirring, until golden and any liquid in skillet is evaporated. Preheat oven to 375°F. On a lightly floured surface roll out 1 dough rectangle into a 16 x 6-inch rectangle and fit it into a 13 1/2 x 4 x 1-inch tart pan with a removable fluted rim. Roll a rolling pin over pastry to trim it flush with top of rim and prick bottom of shell in several places with a fork. Make 1 more tart shell in another tart pan in same manner. Line shells with foil and fill with pie weights or raw rice. Bake shells in middle of oven 10 minutes. Carefully remove foil and weights or rice and bake shells until pale golden, about 5 minutes more. While shells are baking, whisk together goat cheese, butter, and sour cream until smooth and whisk in eggs until combined well. Season custard with salt and pepper. Spread onions evenly in shells and pour custard over onions. Sprinkle thyme over custard and bake tarts in middle of oven 20 minutes, or until puffed and golden. Cool tarts in pans on racks (filling will deflate). Tarts may be made 2 days ahead, cooled completely in pans, and chilled, covered, in pans. Reheat tarts in pans, uncovered, in a 350°F. oven about 15 minutes to crisp crusts. Remove rims from pans. Cut tarts crosswise into 3/4-inch-wide slices to make about 32 hors d'oeuvres and, if desired, halve slices crosswise to make about 64 hors d'oeuvres. Serve tarts warm or at room temperature.