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1
You will also need an instant read or candy thermometer.
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2
Put oven rack in middle position and preheat oven to 400F; Roast squash, cut sides down in a lightly oiled shallow baking dish until neck is tender, about 1 hour; remove from oven and reduce oven temperature to 325F; Cool squash to warm.
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3
Butter an 8-by-2-inch round cake pan; line with bottom with parchment or wax paper and butter paper.
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4
Scoop flesh from squash, discard skin, and puree in a food processor until smooth, about 45 seconds: Put 2 cups puree into bowl, then whisk in whole eggs, 1 cup half and half, 2 tablespoons cheese, 1 1/4 teaspoons salt and 1/2 teaspoon black pepper until combined.
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5
Pour squash mixture into cake pan and bake in a water bath until just set and a wooden tooth pick comes out almost clean, 45 minutes to 1 hour; remove from water and cool flan on rack 15 minutes, invert a flat plate over flan, then invert flan onto plate and carefully remove parchment paper.
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6
Meanwhile, bring whole sage leaves and remaining cup of half and half just to simmer in a 1-to 2-quart heavy saucepan, then remove from heat and let steep, covered 10 minutes; Remove sage, carefully squeezing leaves to extract liquid, and discard leaves.
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7
Whisk yolks into half and half and cook over moderately low heat, stirring constantly with a wooden spoon, until sauce is thick enough to coat back of spoon and register 170 to 175 degrees on thermometer, about 4 minutes (DO-NOT let boil).
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8
Immediately pour sauce through a fine sieve into a bowl, then add remaining 1/2 cup cheese and chopped sage, stirring until cheese is melted, stir in 1/4 teaspoon salt and 1/2 teaspoon pepper.
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9
Cut flan into wedges with a thin knife, wiping knife clean after each slice; Serve flan with sauce.
