In a large mixing bowl, combine flour, salt, olive oil, and cold water along with 1 egg. Stir until a soft dough forms, then shape it into a ball. Cover the dough with plastic wrap and refrigerate for 30 minutes.

Chop the zucchini, eggplant, and bell pepper into bite-sized pieces. In a medium skillet, heat olive oil over medium heat. Add the chopped vegetables along with minced garlic, dried thyme, and Espelette pepper. Sauté for about 10 minutes. Set aside to cool slightly.

In a separate bowl, whisk together the remaining 4 eggs and milk. Season with salt and pepper, then crumble in the feta cheese, mixing gently.

Preheat your oven to 180°C. Roll out the chilled dough and transfer it to the pan, pressing it into the edges. Layer the sautéed vegetables, add cherry tomatoes and red onion, then pour the egg and feta mixture.

Bake for 40-45 minutes until the filling is set and the top is golden brown. Allow to rest for 10-15 minutes before serving.