The Bojon Gourmet

Sourdough pate brisee:
2 Combine the flour and salt in a medium bowl. Add the butter and work with your fingertips or a pastry blender until the mixture resembles coarse meal with some larger, pea-sized chunks remaining. Gradually add the starter tossing the mixture with a rubber spatula or your hands, pressing it against the sides of the bowl, until the dough barely holds together when squeezed. You may not need all the starter, or you may need to add more. Gather the dough into a ball, put it in a plastic bag, and squash it into a 6" disc. Chill at least 1/2 hour.
3 Place a rack in the lowest position of the oven and place a baking stone on it if you have one. Remove the other racks. Preheat the oven to 400º.
4 On a lightly floured surface, roll the dough into a 12" round. Fit into a 10" tart pan, double over the sides and press to even them out. They should be flush with the top of the pan. Chill at least 1/2 hour.
5 Line the crust with parchment paper and pie weights, dry garbanzos or cleaned pennies (I keep mine in a cheesecloth bag and put the whole thing atop a round of parchment placed in the crust). Press the weights so they snuggle into the corners, and bake on the stone until the bottom is mostly dry, about 10 minutes. Carefully remove the weights and parchment and continue baking until the bottom is dry and firm, another 10 minutes or so.
6 Lower the oven to 375º.
Tart filling:
9 Melt together the butter and olive oil in a medium skillet over medium heat. Saute the leek with 1/4 teaspoon salt until very tender and beginning to brown in places, 10 - 15 minutes. Remove from heat and toss in the truffle oil. Set aside.
10 Sprinkle the gruyere evenly over the bottom of the parbaked tart shell. Sprinkle the leeks over the cheese, then the squash chunks and the goat cheese in marble-sized bits, and finally the sliced sage. Sprinkle 1/4 teaspoon salt over the squash.
11 Beat together the eggs with 1/2 teaspoon salt until very smooth, then beat in the milk and cream.
12 Place the tart on the baking stone and carefully pour in the milk/egg mixture until the tart is as full as possible without overflowing. (A tip from the Tartine cookbook: when the custard has barely set, make a small hole in the center of the tart with a knife, and pour in any extra custard.) Bake at 375º until slightly puffed, set, and browned in places, about 40 minutes.
13 Remove and cool slightly before slicing into wedges and serving.