Chicken Pot Pie

I make this recipe all the time.  It is one of my main staples that I make about twice a month. I have made this recipe at the cottage and I used a 2.8L casserole dish. The casserole dish worked, but some of the filling did spill over. It is best to use the 3L dish as suggested. I sometimes make the filling ahead of time and I add the puff pastry top just before cooking. This recipe is taken from "The Best of the Best and More" by The Best of Bridge.

1/4 cup butter
1/4 cup flour
salt & pepper to taste
2 Tbsp. finely chopped onion
3 cups chicken broth
2 carrots, chopped in small pieces
2 celery stalks, chopped in small pieces
2 potatoes, cubed in small cubes
3 cups sliced mushrooms
2 Tbsp. butter
1/2 cup peas
3 cups cooked & diced chicken
Pastry to cover 3-Qt. (3L) casserole or frozen puff pastry dough

1. Melt butter in large saucepan over medium heat. Blend in flour, salt, pepper and onion. Gradually stir in chicken broth. Cook, stirring constantly, until smooth and thickened. Add carrots, celery and potatoes. Cook until fork tender. 
2. In a small frying pan, cook the mushrooms in butter. Add mushrooms, peas and chicken to vegetable mixture. Mix well and pour into large casserole. Cover with rolled pastry and slash to allow steam to escape. 
3. Bake in preheated 400F (200C) oven for about 45 minutes, or until pastry is golden. If pastry becomes too brown, cover loosely with foil.