1. Pulse flour, sugar, and salt in a food processor to combine. Add butter and process until mixture resembles fine crumbs. Combine egg yolk and water in a cup; with machine on, add through feed tube and process just until mixture holds together. Shape dough into a ball; flatten into a thick disk. Wrap well in plastic wrap and refrigerate 30 minutes or overnight.
2. Roll pastry between 2 sheets of lightly floured waxed paper into an 11-inch circle. Fit into a 9-1/2-inch tart pan with removable bottom. Fold and press overhang in against sides. Freeze until ready to use.
3. Arrange rack in lower third of oven. Heat oven to 375 degrees F.Make filling::
4. Bake squash in a covered baking dish until very tender, 30 minutes, or microwave in a shallow 2-quart microwaveproof dish 15 minutes. Press through a sieve into a medium bowl. Makes 1-1/4 cups.
5. Combine butter, 2 tablespoons maple syrup, and the pecans in a pie plate, stirring until pecans are well coated. Spread pecans in a single layer and bake 10 minutes, turning nuts occasionally to coat. Cool in pan on a wire rack.
6. Meanwhile, line pastry with foil and fill with dried beans or pie weights. Bake 10 minutes. Remove foil and beans. Brush inside of pastry with egg white and bake 5 to 8 minutes more, until golden. Cool on wire rack.
7. Reduce oven temperature to 350 degrees F. Whisk remaining 1/3 cup maple syrup, the heavy cream, sugar, eggs, and nutmeg into squash, until well mixed. Place tart pan on a cookie sheet on oven rack; carefully pour squash mixture into pastry. Bake 20 minutes. Remove from oven and arrange glazed pecans around edge. Return to oven and bake 10 to 15 minutes more, just until set. Cool completely on wire rack. Serve with Maple Whipped Cream.
1. Beat heavy cream in a small mixer bowl on medium-high speed to soft peaks. Slowly beat in maple syrup; beat to stiff peaks (do not overbeat).