For the crust
1 cup flour, plus more for rolling out dough
1/2 teaspoon salt
1/2 teaspoon freshly ground black pepper
7 tablespoons butter, chilled and cut into small pieces
For the filling
3 tablespoons butter
1 leek, white and very light green parts halved, cleaned, and thinly sliced
1/2 teaspoon salt
1/2 pound chanterelles, cut into about 1-in. pieces
2 teaspoons fresh thyme leaves
3 tablespoons heavy whipping cream
1/4 teaspoon freshly ground black pepper
1 cup grated gruyère cheese
Make crust: In a medium bowl, mix flour, salt, and pepper. Rub butter into flour until most of it looks like cornmeal but some larger, pea-size pieces remain. Drizzle in 3 tablespoons ice-cold water while stirring quickly with a fork. Or, pulse flour, salt, pepper, and butter in a food processor until a coarse, cornmeal-textured mixture forms, then drizzle in ice water until dough comes together. Turn dough onto a large piece of plastic wrap and use wrap to press dough into a disk. Wrap in plastic wrap and refrigerate at least 30 minutes and up to 2 days.
Preheat oven to 375F. Lightly flour a work surface. Unwrap dough; with a rolling pin gently roll into a 9-inch circle. Place rolled-out dough in a 7 inch tart pan. Trim edges flush with pan edges. Cover dough with a large piece of aluminum foil and weigh down with pie weights. Bake crust 20 minutes. Lift foil and weights off crust and bake until beginning to turn golden, about 10 minutes. Let crust cool to room temperature.
Meanwhile, make filling: Melt butter in a large frying pan over medium-high heat. Add leeks and salt and cook, stirring, until leeks are soft, about 3 minutes. Turn heat to high and add mushrooms. Cook, stirring constantly, until mushrooms have given off their liquid, 5 to 10 minutes. Stir in thyme, cream, and pepper, then turn off heat and let cool to room temperature. Spread half of the grated cheese over tart crust. Spread leek-mushroom mixture on top and sprinkle with remaining cheese. Bake until cheese is melted and golden, about 25 minutes.
Let rest for a minimum of 10 minutes before cutting. Can be served hot or at room temperature.