2. Add the warm milk to the butter- flour roux in a slow steady stream, stirring constantly. The slow addition and warm milk will help you to avoid lumps, but if you do see lumps of roux in the milk, continue stirring, pressing down with the back of your spoon or spatula to break these up and incorporate them into the milk.
3. Continue to stir the mixture over medium heat until it begins to thicken and just come to a bubble. Immediately remove pan from the heat and set aside to assemble the pie.
4. Preheat the oven to 400 degrees.
5. Add to a 2-quart casserole dish the turkey, peas, carrots and pearl onions. Toss together to distribute evenly. Pour over this the béchamel sauce and stir to combine.
6. Remove the puff pastry from the refrigerator and roll this sheet out to be slightly larger than the top of the casserole dish. Trim off any edges necessary to fit the pastry into the pan with a little bit of over hang. With the tip of a sharp knife cut a few slits in the crust so that the stream may escape while baking.
7. Using a pastry brush, brush the egg wash lightly and evenly over the surface of the pastry.
8. Place the dish in the preheated oven and bake for 35-45 minutes, or until the crust is deep golden brown and the sauce is bubbling.
Recipe courtesy of food blogger and novelist Jane Ward, 2012.