Make the crust first. Combine flour and salt. Cut in the butter and shortening with a dough cutter or a fork until it looks like chunky breadcrumbs. Add the vinegar then drizzle the ice cold water, mix until the dough holds together into a ball. Keep adding iced water if necessary. Form the dough into a ball. Wrap in a saran wrap then store in the fridge for 30 minutes or so.
Make the sauce. Melt butter in a saucepan. Whisk in the flour until bubbly. Add the stock, then let it simmer in medium heat while whisking continuously until it boils. Cook for another 2 minutes or so then whisk in the cream or milk. Season with salt and pepper. Take it out of the stove and set aside.Preheat the oven to 355°. Make the filling. Melt butter in a nonstick pan. Add onions to cook until translucent. Add the chicken to cook. Then add the carrots to cook until soft. Add the celery and and mushrooms to cook for about 5 minutes. Stir in the peas.
Slowly pour in the sauce, while stirring gently. Season with salt and pepper.
Carefully transfer the whole thing into a rectangular baking dish.
Get the dough from the fridge. Sprinkle flour in a counter to prevent the dough from sticking. Roll out the dough slightly larger than the baking dish. Carefully lift the dough using the rolling pin then place it on top of the baking dish. Crimp the edge and be decorative as you want.Brush the top with a beaten egg. Cut a few vents in the top of the crust. Bake it in the oven to cook for 30-35 minutes or until the crust is golden brown.
