01 - Combine flour and salt in a large bowl. Cut in cold butter using a pastry cutter or your fingers until mixture resembles coarse crumbs. Gradually add ice water, mixing until dough just comes together. Shape into a disk, wrap in plastic, and refrigerate for at least 30 minutes.
02 - Heat olive oil in a skillet over medium heat. Add leek and sauté for 2 minutes. Add asparagus and zucchini, cooking for another 3–4 minutes until just tender. Stir in peas, garlic, and thyme; cook 1 minute more. Remove from heat and let cool slightly.
03 - Preheat oven to 400°F. Line a baking sheet with parchment paper.
04 - Roll out the chilled dough on a lightly floured surface into a 12-inch circle. Transfer to the prepared baking sheet.
05 - In a bowl, combine the cooled vegetables, goat cheese, lemon zest, salt, and pepper.
06 - Spoon the filling onto the center of the pastry, leaving a 2-inch border. Fold the edges of the dough over the filling, pleating as needed, leaving the center exposed.
07 - Mix the beaten egg with milk and brush over the pastry edges.
08 - Bake for 30–35 minutes, or until the crust is golden and the filling is bubbling.
09 - Let cool for 5 minutes before slicing. Garnish with fresh herbs if desired.