Add potatoes, carrots, and celery to a medium stockpot. Add just enough water to cover and bring to a boil. Reduce heat and simmer 15 minutes, or until potatoes are fork tender. Drain.
Add butter and onions to a saute pan. Cook until onions are translucent. Add flour and salt and pepper and stir until combined.
Add milk and chicken broth (slowly) and whisk until completely incorporated. Simmer on low heat until sauce thickens.
Pour cream sauce over vegetables. Stir in frozen peas and chicken.
Ladle the chicken pot pie filling into 8 oven-proof ramekins.
Roll out pie crust. Cut out circles the same size as your ramekins.
Sprinkle the chicken pot pie crust with seasoning salt and bake @ 425 degrees for 30 minutes or until crust is golden brown. Cool 15 minutes before serving.