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1
Preheat the oven to 400.
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2
Halve the squash lengthwise and remove the seeds.
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3
Place the squash, cut side up, in a baking pan.
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4
Season with 1/2 teaspoon each of the salt and pepper and cover tightly with foil.
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5
Bake for about 1 hour, until the squash are tender but not mushy.
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6
Let cool slightly.
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7
Meanwhile, in a medium saucepan, combine the leek, olive oil and 2 teaspoons of water.
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8
Cover and cook over moderately low heat until the leek is soft and translucent, about 5 minutes.
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9
Uncover and stir in the wine.
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10
Increase the heat to high and boil until the liquid is reduced to approximately 3 tablespoons, about 3 minutes.
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11
Stir in the stock, milk, sugar and remaining 1/4 teaspoon each salt and pepper.
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12
Remove from the heat.
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13
Using a big spoon, scoop the flesh from the squash in large pieces.
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14
Place in a medium bowl.
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15
To assemble the gratin, preheat the oven to 400.
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16
Bring the leek mixture to a boil.
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17
Spoon half of the squash into a 6- to 8-cup casserole.
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18
Ladle half of the leek mixture over the top and cover with half of the toast and half of the Gruyere.
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19
Repeat the layers with the remaining squash, leek mixture, toast and Gruyere.
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20
Sprinkle the Parmesan cheese over the top.
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21
Bake the gratin for 30 minutes, or until the top is browned and bubbly.
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22
Garnish with the basil and serve.
