For the pastry:
1 1/4 cups all-purpose flour
1/4 teaspoon salt
1 stick unsalted butter, cut into pieces
1/4 cup sour cream
2 teaspoons fresh lemon juice
1/4 cup ice water
For the filling:
1 small butternut squash (about 1 lb)
2 Tablespoons olive oil
1-2 Tablespoons butter
1 large onion, halved and thinly sliced into half moons
1 teaspoon salt
Pinch of sugar
1/4 teaspoon cayenne
3/4 cup fontina cheese grated or cut in small bits
1 1/2 teaspoons fresh sage leaves
1. Make pastry: In a bowl, combine flour and salt. Place the butter in another bowl. Place both bowls in the freezer for 1 hour. Remove the bowls and make a well in the center of the flour. Add the butter to the well and use a pastry blender to cut it in until the mixture resembles coarse meal. Make another well in the center. In a small bowl, whisk together sour cream, lemon juice, and water and add half of this mixture to the well. With your fingertips, mix the liquid until large lumps form. Remove the lumps and repeat with the remaining liquid and flour. Pat the lumps into a ball; do no overwork the dough. Cover with plastic wrap and refrigerate for 1 hour.
2. Prepare squash: Preheat oven to 375 degrees. Peel squash, then half and scoop out seeds. Cut into a 1/2 inch disk. Toss pieces with olive oil and 1/2 teaspoon of salt. Roast on foil lined sheet for 30 minutes, turning midway through. Set aside to cool slightly.
3. Caramelize onions: Melt butter in heavy skillet and cook onion over low heat with 1/2 teaspoon salt and pinch of sugar, stirring occasionally until soft and lightly golden brown, about 20 minutes. Stir in cayenne.
4. Raise the oven temperature to 400 degrees. Mix squash, onions, cheese, and herbs together in a bowl.
5. Assemble: On a floured work surface, roll the dough out thin. Cut the dough into small squares. Place a spoonful of squash mixture in the middle of each square. Fold the boarder over the squash mixture, bringing corners to meet in the middle. Repeat until dough is gone. You may have some left over filling.
6. Bake until golden brown, 20-25 minutes. Cut into wedges and serve hot, warm, or at room temperature.