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1
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.
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2
Add 2 tablespoons ice water and toss with a fork or pulse until incorporated.
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3
Add enough remaining ice water, 1 tablespoon at a time, tossing with fork or pulsing to incorporate, to form a dough.
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4
On a work surface smear dough with heel of hand in 3 or 4 forward motions to make dough easier to work with.
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5
Divide dough in half and pat each half into a rectangle, about 6 by 3 inches.
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6
Chill rectangles, wrapped separately in plastic wrap, at least 1 hour and up to 1 week.
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7
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.
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8
Cook onions, uncovered, stirring, until golden and any liquid in skillet is evaporated.
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9
Preheat oven to 375F.
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10
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.
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11
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.
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12
Make 1 more tart shell in another tart pan in same manner.
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13
Line shells with foil and fill with pie weights or raw rice.
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14
Bake shells in middle of oven 10 minutes.
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15
Carefully remove foil and weights or rice and bake shells until pale golden, about 5 minutes more.
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16
While shells are baking, whisk together goat cheese, butter, and sour cream until smooth and whisk in eggs until combined well.
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17
Season custard with salt and pepper.
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18
Spread onions evenly in shells and pour custard over onions.
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19
Sprinkle thyme over custard and bake tarts in middle of oven 20 minutes, or until puffed and golden.
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20
Cool tarts in pans on racks (filling will deflate).
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21
Tarts may be made 2 days ahead, cooled completely in pans, and chilled, covered, in pans.
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22
Reheat tarts in pans, uncovered, in a 350F.
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23
oven about 15 minutes to crisp crusts.
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24
Remove rims from pans.
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25
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.
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26
Serve tarts warm or at room temperature.
