Preheat oven to 350°F (180°C).

Cut chicken into bite size pieces.

In a frying pan, heat 1 Tbsp (15 mL) of oil. Add chicken and poultry seasoning to the pan and stir frequently until chicken is cooked through. Place cooked chicken in a bowl and set aside. In the same frying pan, heat 1 Tbsp (15 mL) of oil. Add the carrots, celery, leeks and mushrooms and sauté until vegetables are just slightly cooked. Remove pan from heat and set aside.

In a saucepan, melt butter on medium-low heat. Whisk the flour into the butter. Add Dijon mustard and mix thoroughly. Add white wine, chicken broth and cream and whisk together. Reduce heat slightly and add salt and pepper to taste. Remove mixture from heat once it has thickened. If sauce is too thick, add a little more chicken broth.

In a 3 quart (2.8 L) casserole dish, add chicken and vegetables. Add the sauce and stir together.

Place thawed puff pastry sheet on top of the casserole dish, making sure to cover the edges. With a knife, cut small lines, approximately 1 inch (2.5 cm) into the pastry. Brush the pastry with beaten egg and place pot pie in the over. Bake for about 30 minutes or until a meat thermometer inserted into the centre of the pie reads 165°F (74°C).

Serve with mashed potatoes and a garden salad.