Make the Smoked Bacon Pastry in advance or purchase one. Bring a large pot of lightly salted water to a boil over high heat. Drop in the potatoes and carrots and cover until the boil returns. Boil for 2 minutes; drain. Turn the vegetables into a large bowl, add the frozen peas, and toss (the peas will thaw and cool the vegetables slightly at the same time). Add the diced chicken and parsley. Melt the butter in a large saucepan over moderate heat. Add the onion and saute to soften, 3 to 5 minutes. Add the garlic and cook for 1 minute longer. Stir in the flour and cook for 2 minutes; the mixture will be dry. Pour in the chicken stock and stir or whisk constantly until it comes to a simmer. In a small bowl, combine the sherry and cornstarch and stir to dissolve. Pour into the sauce and stir constantly until slightly thickened, 1 to 2 minutes. Add the tarragon, nutmeg, salt and pepper. Remove from the heat and stir in the lemon juice. Pour the mixture over the chicken and vegetables and stir to combine. Turn into the casserole and stir occasionally until cooled to room temperature. Preheat the oven. On a lightly floured surface, roll out the pastry into a rectangle about 14 x 10 inches or slightly larger. Carefully pick up and transfer the pastry to the top of the pie filling. Roll the edge of the pastry to fit the pan and flute decoratively against the sides of the pan. In a cup, stir together egg yolk and 1/2 teaspoon of water. Brush the glaze over the top of the pastry, but not the fluted edge. Cut 4 to 5 steam holes in 3 places across the top. Place on a sheet of aluminum foil to to catch any drips and bake for 45 to 55 minutes, or until the crust is golden brown and the filling is bubbly. Let stand for at least 10 minutes before serving.