For the pastry shell: Cut butter into pieces. In a food processor blend flour, sugar, and salt until combined. Add butter and pulse until most of mixture resembles coarse meal, with remainder in small pea-sized lumps. Add 2 tablespoons of the water and pulse just until incorporated. Test mixture by gently squeezing a small handful: If it does not hold together, add enough remaining water, 1 tablespoon at a time, pulsing and testing, until mixture just forms a dough.

Form dough into a disk. Chill dough, wrapped tightly in plastic wrap, 30 minutes.

On a lightly floured surface roll out dough into a 12-inch round (about 1/8 inch thick). Fit dough into an 11'' tart pan with a removable fluted rim. Freeze pastry shell 20 minutes.

Preheat oven to 375° F.

Line pastry shell with foil and fill with pie weights or raw rice. Bake shell in middle of oven until edge is pale golden, about 20 minutes. Carefully remove foil and weights and bake shell 10 minutes more, or until bottom is golden. Leave oven on. Cool shell in pan on a rack.

For the filling: Halve the squash and scoop out seeds. Lightly brush each cut side of squash with about 1 teaspoon oil and lay cut sides down on a baking pan. Roast in middle of oven for about 40 minutes, or until soft.

While squash is roasting, place heavy skillet over medium heat. Add 1/2 tablespoon butter and remaining 1.5 teaspoons oil, plus the onion. Stir occasionally, until onion is soft and just golden brown, about 15 to 20 minutes. If the onion starts to darken before it is softened, turn the heat down a bit.

Cool squash and scoop out flesh. In a food processor purée squash. Add whole egg, egg yolk, and cream, and blend well. Transfer mixture to a large bowl and stir in cheeses, herbs, onion, salt, and pepper to taste. Pour filling into shell, smoothing the top.

In a small bowl, melt remaining tablespoon butter in the microwave. Stir in bread crumbs until combined well. Sprinkle bread crumb mixture evenly over filling. Bake tart in middle of oven 40 minutes, or until filling is set. Cool tart in pan on rack 10 minutes and carefully remove rim. Serve while still warm or at room temperature. It will be best on the day it is made.