In a large mixing bowl, whisk together the flour and salt. Cut in the cold butter using a pastry blender or your fingers until the mixture resembles coarse crumbs. Gradually add ice water, 1 tablespoon at a time, mixing until the dough just comes together. Form the dough into a disk, wrap in plastic wrap, and refrigerate for at least 30 minutes.

In a medium saucepan, melt butter over medium heat. Add chopped onion and cook until softened, about 3-5 minutes. Stir in condensed cream of chicken soup, milk, cooked chicken, mixed vegetables, thyme (if using), salt, and pepper. Heat through and simmer for 5 minutes, stirring occasionally, until slightly thickened. Remove from heat and let cool slightly.

Preheat oven to 375°F (190°C). On a lightly floured surface, roll out the pastry dough to about 1/8 inch thickness. Cut out circles or squares to fit your desired pastry size. Place a spoonful of chicken filling in the center of each pastry. Fold the pastry over the filling to form a half-moon or square shape, crimping the edges to seal. Place pastries on a baking sheet lined with parchment paper. Brush the tops with egg wash. Bake for 20-25 minutes, or until golden brown and pastry is cooked through.

Let the chicken pastries cool slightly on the baking sheet before serving. Serve warm and enjoy!