Instructions for the tart crust
Preheat oven to 375. Add flour and salt to a food processor. Pulse 2 to 3 times until combined.
Chop butter into rough cubes and scatter over flour and pulse 4 to 5 times until flour is evenly distributed. (Dough should look crumbly.)
Add 3 Tbsps ice water through the tube pulsing once each time. The crumbs should begin to form larger clusters. Add the vodka and continue to pulse until the dough comes together in a ball and gets awkward spinning in the machine.
Remove dough from the bowl and place in a mound on a clean surface. Work the dough just enough to form a ball – create a disc and wrap with plastic wrap and refrigerate for at least 1 hour and up to 2 days (If I am in a hurry, which I always am, I will freeze the dough for 30 minutes)
On a lightly floured surface, roll out one dough disc to a 15-inch round circle about 1/8 inch high. Check for size by inverting tart pan over dough round. Look for a 1-inch edge around the tart pan. Carefully press the dough into the pan mounding the sides up a bit higher than the pan by folding over excess dough. Press the tines of a fork into the pastry down to the pan about 12 times with a fork to “dock” the crust. Place tart in the freezer while you make the filling to get set for 15 minutes. Transfer the tart pan to a baking sheet. Set in oven and bake at 375 until the crust is dry and set up and starting to pull from the sides, 13 to 15 minutes.
Instructions for the filling
Heat oil in a large skillet over medium-high heat. Add onions and Swiss chard stems and cook, stirring often until softened and starting to brown, 8 to 10 minutes. Stir in salt, pepper, and fresh thyme. Add chopped Swiss chard leaves a handful at a time and wilt. Add 2 Tbsp water, cover, and cook until the stems are tender, and the Chard leaves are completely cooked down for about 5 minutes.
Make the custard: Puree eggs and milk and one 4 oz goat cheese log in a blender until smooth.
Instructions for Tart assembly
Reduce oven temperature to 350 degrees. Brush mustard over the bottom of the par-baked tart crust. Spread the Swiss Chard Filling out over the crust. Pour the Goat Cheese custard over the filling and garnish with the leftover hunks pulled from the remaining goat cheese log.
Bake the tart on the baking sheet until the shell is browned, the filling is set up, lightly golden, and puffed, for about 40 minutes. Let cool on a wire rack 10 minutes before removing tart pan sides.