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1
Make crust: Preheat oven to 400 degrees F (220C).
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2
Roll out dough 1/8 inch thick on a lightly-floured surface and fit it into a 9 1/2 inch square flan form set on a baking sheet, leaving a 1/4 inch overhang.
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3
Fold overhang inward onto the sides of the shell and press firmly against the flan form.
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4
Prick shell lightly with fork and chill for 30 minutes or freeze for 15.
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5
Line pastry with foil and fill with pie weights.
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6
Bake in lower third of oven for 15 minutes.
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7
Remove weights and foil and bake shell a further 5-8 minutes, or until golden.
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8
Let cool.
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9
Make filling: In a large skillet, heat oil over moderately-high heat until hot but not smoking.
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10
Stir in onions, garlic and season to taste.
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11
Cook mixture, covered, over low heat, stirring occasionally, for 20-25 minutes, or until onions are softened.
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12
Add sage, thyme and cream, bring to boil, and simmer, stirring occasionally, for 3-5 minutes, or until thickened.
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13
Let onion mixture cool.
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14
(may be made 1 day ahead) Spread onion evenly into pastry shell.
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15
Peel and halve apple lengthwise, core, and slice very thin crosswise.
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16
Arrange apple and squash slices decoratively, overlapping, covering onion.
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17
Season with salt and top with butter.
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18
Preheat oven to 375 degrees F (190C).
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19
Bake in upper third for 15 minutes.
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20
Cover tart with foil and bake for 30 minutes more, or until squash is tender.
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21
Let tart cool for 20 minutes before removing from flan form.
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22
Serve warm or at room temperature.