Whirl flours and ¼ teaspoon salt in a food processor until mixed. Add butter pieces. Pulse until butter forms pea-size pieces. Add water and egg yolk; pulse to form a dough. Turn dough out onto a lightly floured surface, and knead gently several times, adding a little more water if needed. Form dough into a ball and flatten into a disk. Wrap in plastic, and refrigerate for at least 20 minutes.

Remove leaves from thyme sprigs. In a skillet, warm olive oil. Add thyme leaves, onions and leek, and cook over medium-low heat, stirring occasionally, until softened, about 10 minutes. Remove from heat. Season with pepper and ½ teaspoon salt. Let cool a bit, and then stir in ¼ cup sorrel.

Preheat oven to 425°. Place dough between two parchment papers. Roll into a rough 12- to 13-inch circle about ¼-inch thick. Remove top parchment. Place dough with bottom parchment onto a baking sheet. Spread cheese onto pastry, leaving a 2-inch border. Spread onion mixture on top. Fold border up.

Bake tart for 15 minutes. Reduce oven heat to 350°, and bake until crust is golden, about 20–25 minutes. Just before serving, top tart with remaining 3 tablespoons sorrel.