Homemade Chicken Pot Pie

1 medium potato, diced
2 celery stalks, chopped
1/2 onion, diced
2 tbl. vegetable oil
4 TBL butter
1/4 cup all-purpose flour
1 cup milk
1 1/2 cups chicken broth ( I used a whole can of chicken broth & less milk)
1 tsp. dried thyme
1/2 tsp. garlic powder (I used garlic salt)
1/2 tsp. ground white pepper.
2 cups cooked chicken (leftover turkey is great too)
1 bag mixed veggies (green beans, lima beans, carrots, peas, corn) from Publix
1 double pie crust 
In skillet, heat the oil and put in the celery and onions and potato, cooking til tender (the potato will not be completely cooked through). Add to baking dish and set aside.
Add the butter in the same skillet and melt, then add the flour and stir, making a paste. Stir in the chicken broth, keep whisking until it starts to thicken, turn it up and it will start to boil. Keep whisking. Add the milk . Whisk until it is thickened sufficiently. This part takes about 10 min. or so.
Preheat oven to 400*.
Place the chopped chicken (or turkey) in with the sauteed onion and celery in the baking dish. Add the frozen bag of mixed veggies. Mix all of that up well. Pour the thickened broth, making sure it seeps down in the veggies and chicken.
Fold out the piecrust, stretching it from side to side and crimp down on the edges so it covers the entire dish. You can make it all fancy with a fork. Cut a couple of slits in the crust.
Bake for about 40-45 minutes until the top is golden brown.