1/2 cup (1 stick) unsalted butter, cut into tablespoons
1/4 cup ice water
First, make the shell: In the bowl of a food processor fitted with a steel blade, whirl flour and salt. Add butter chunks and pulse until mixture looks like rough cornmeal. Add 1/4 cup ice water and pulse until dough begins to come together. Remove from processor and gather into a ball. Press ball into a flat disk, wrap in plastic, and refrigerate at least 30 minutes before rolling out.
Preheat oven to 450° and set a rack to the middle position. Roll out tart dough and transfer to a 9- or 10-inch tart pan with removable bottom (or a 9- or 10-inch pie plate). Prick dough all over with a fork.
Cover with foil and fill with pie weights or dry beans. Bake 10 minutes. Lower heat to 400° and carefully remove foil and weights or beans. Continue baking until crust is golden brown, about 10 minutes. Remove from oven and cool slightly.
For the filling:
3 medium-size 'Yukon Gold' potatoes, scrubbed but not peeled
2 medium-size ripe tomatoes, cut into 1/4-inch-thick slices
1-1/2 cups shredded aged Gouda
Kosher or sea salt
Freshly ground black pepper
2 teaspoons minced fresh Italian parsley
1 teaspoon fresh thyme leaves
3/4 cup whole milk or half-and-half
2 large eggs, lightly beaten
Dash cayenne pepper
Garnish: fresh thyme sprigs
Meanwhile, make the filling: Cover potatoes with cold water in a saucepan and bring water to a boil. Boil until you can easily pierce potatoes with a fork (they shouldn't be completely cooked or falling apart), about 10 minutes. Drain. When cool enough to handle, cut into 1/4-inch-thick slices.
Layer potato slices, tomato slices, and Gouda into the baked tart crust, sprinkling with salt, pepper, and herbs as you go. You'll get about 2 layers of each. Finish with a little cheese.
In a medium-size bowl, whisk together milk or half-and half, eggs, and cayenne pepper, and pour over all. Top with thyme sprigs.
Bake until top is set and cheese is bubbling, 35 to 40 minutes. Let sit 10 minutes before cutting into wedges.