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1
Preheat oven to 425 F. and lightly grease 2 large baking sheets.
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2
Slice enough of red and yellow onions to measure 5 cups each and in a large skillet cook in 3 tablespoons oil with salt and pepper to taste, covered, over moderate heat, stirring occasionally, until golden and tender, about 15 minutes.
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3
Stir in cream and cool.
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4
Trim remaining onions and cut lengthwise into sixths, keeping wedges intact, and arrange, narrow sides up, 1/2 inch apart on baking sheets.
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5
Sprinkle onion wedges with salt and pepper to taste and roast in middle and lower thirds of oven 20 minutes, or until tender.
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6
Cool onion wedges.
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7
On a lightly floured surface with a lightly floured rolling pin roll out 1 pastry sheet into a 13-inch square.
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8
Using an inverted 6-inch plate as a guide, cut out four 6-inch rounds, discarding scraps.
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9
Cut out 12 more rounds from remaining 3 pastry sheets in same manner to make a total of 16 rounds.
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10
Fold in edge of each pastry round to form a 1/4-inch-wide border.
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11
(If pastry becomes too soft to work with, chill until firm.)
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12
Transfer rounds to 2 large baking sheets and chill until firm, about 10 minutes.
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13
Top each round evenly with a scant 1/4 cup sliced onion mixture.
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14
Arrange roasted onions on their sides decoratively (alternating yellow and red) on top of tarts and season with salt and pepper.
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15
Bake tarts in middle and lower thirds of oven, switching positions of pans halfway through baking, 20 to 25 minutes, or until bottoms are golden brown and roasted onions are very tender.
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16
Transfer tarts to racks to cool.
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17
Tarts may be made 1 day ahead and chilled, covered loosely with plastic wrap.
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18
Bring tarts to room temperature before serving or heat in a preheated 350 F. oven until heated through.
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19
Brush tarts with additional oil and serve whole or halved.
