Cut the onion into tiny pieces and place in a large saute pan with 1 tablespoon of the butter. Fry until translucent and remove from pan.
Add the rest of the butter to the pan and melt. Then add the flour, mixing to combine. Cook for about 1 minute or until the flour butter mixture starts to brown. Slowly add the milk, a little at a time to make your roux. Mix in between each milk addition until you have a smooth sauce.
Add the bullion, black pepper, and cajun seasoning, and then add the water in the same way you did the milk until you have a nice thick sauce.
Add the peas and corn and chicken and mix to combine, turning off the heat.
Roll out the pie crusts on a lightly floured surface, one at a time. If you’re using small pie pans (<8in diameter) use one crust per pie. For large pans you will use 2 crusts.
Place the crust in the bottom of the pan and fill with pot pie filling. Place a second pie crust on top for a large pie, or fold the sides up to meet on the top for a small pie, pinching to seal the dough together. Cut 3-5 slits in the top for the pie to vent when cooking.
Bake in a 400F oven for 35-45 minutes or until the crust is golden brown and the inside is bubbly.