Preheat oven to 375 Degrees Farenheit.
In a large pot on high heat, pour the chicken broth and bring it to a boil. To this, add the bay leaves, carrots and potatoes and reduce heat to medium-high. Allow this mixture to simmer for 30 mins or until potatoes and carrots are fork tender. Drain but conserve half of the liquid, remove the bay leaves and set aside.
To a large pan on medium high heat, add the olive oil, diced chicken, 2 Tbsp garlic salt and 1 Tbsp of the pepper and cook until lightly browned (chicken should be fully cooked). Remove chicken from pan and set aside.
To the large pan (where the chicken was cooked) on medium heat, add 1 Tbsp of the butter, the diced onion, remaining garlic salt and pepper. Cook until the onions become translucent. Add the musrooms and white wine and allow the mixture to reduce and thicken (or until the wine has evaporated). Add the peas, carrot and potato mixture and sautee until potatoes are lightly browned. Remove from heat.
In a medium pot, on medium heat, add the remaining butter and allow to melt. To the melted butter, add the flour and whisk constantly for 2-3 minutes.
Next, increase the heat to medium high and add the remaining chicken stock (about 4 cups) that was conserved earlier and milk and whisk constantly until the mixture thickens and begins to bubble. Remove from heat. Stir in the cream cheese, cheddar cheese, brown mustard and salt and pepper to taste.
In a very large bowl, combine the potato mixture, chicken and cheese sauce and set aside.
Roll out both sheets of the puff pastry to a 12 inch square and place 1 sheet into a 10X10 pan and press.
Use a fork and gently poke the puff pastry (so that this bottom piece with not rise. Add the Potato/Chicken mixture and top with second puff pastry, making sure to seal the edges. Cut 2 small slits in the top and brush with the egg.
Place in the oven and cook for 1 hour.