Step 1 Preheat oven to 180°C. Spray a ceramic pie dish with oil.
Step 2 Spray a frying pan with oil and place over medium heat. Add chicken and cook, turning, until cooked through. Remove and chop into 1cm pieces. Set aside.
Step 3 Add onion, leek, carrot and garlic to pan. Cook, stirring, until soft. Add mushrooms and cook for 5 minutes. Return chicken to pan.
Step 4 Place milk, flour and stock powder into a bowl and whisk to combine. Add mixture to pan and stir constantly until mixture thickens.
Step 5 Spoon mixture into pie dish. Brush a little egg white around top edge of dish then position pastry over top. Trim off edges, then glaze pastry with remaining egg white. Pierce a few holes in the pastry using a fork. Bake for 15 minutes, or until pastry is puffed and golden. Serve with steamed green beans.