Cook the Book: Chicken and Leek Pasties

2 cups all-purpose flour

A pinch of sea salt

2/3 cup chilled unsalted butter, cut into small cubes

For the filling:

2 tablespoons unsalted butter

2 to 3 leeks (about 1 pound), trimmed and finely sliced

1 teaspoon coarsely chopped fresh thyme leaves

2/3 cup heavy cream

1 teaspoon English mustard

Sea salt and freshly ground black pepper

About 12 ounces boned chicken thigh and breast meat (or leftover cooked chicken), cut into thick slices

1 tablespoon canola or olive oil (if using fresh chicken)

To finish:

1 egg, lightly beaten with

1 teaspoon milk, for glazing