Preheat oven to 375 degrees.
In a large saucepan melt butter over medium heat. Add peas, carrots, and celery and cook for 3-4 minutes while stirring frequently. Add chicken and cook for an additional minute.
Add flour and stir until absorbed into vegetables and chicken. Cook for 1 minute. Stir in chicken broth and bring to a boil and cook until it begins to thicken, approx. 3-4 minutes.
Season to taste with salt, pepper, and onion powder.
Add half and half and boil until it begins to thicken, approx. 3-4 minutes. Stir in buffalo sauce and remove from heat.
Scoop half of the chicken mixture into a pie plate and top with half of the blue cheese. Repeat with remaining chicken mixture and cheese. Top with pie crust.
Make an egg wash by combining 1 tbsp. water with the beaten egg. Brush over crust. Bake until bubbly and crust has browned, approx. 30-35 minutes. Let cool slightly before serving.