preheat your oven to 400° (or other temp according to package directions if using store bought pastry)
In a medium pot, bring some water (or chicken stock) to a simmer and place chicken breasts in it to cook. This should take about 15-20 minutes *Do not boil as this will toughen the chicken. Then remove and let cool before dicing into 1" cubes
While the chicken is cooking, dice your vegetables
In a large saucepan melt 2T of the butter and saute the onion, celery and carrots (if your carrots are large you can microwave them in some water to par cook them, then add them to the mix).
Cook veggies until they're softened about 3-5 minutes, season with Salt and pepper
Add 4 further Tbsp of butter and once it is melted, sprinkle with the flour to make a roux and cook for 2 minutes, stirring constantly.
Add 1½ cups of milk and 3 cups of stock and simmer on med-hi, stirring, until the mixture thickens (5-8 minutes)
Add Additional seasonings, thyme, poultry seasoning, salt and pepper to taste
Add in the chicken (chopped) and 2 cups of mixed frozen vegetables, stir and let simmer for a couple of minutes further
If making pies, let the mixture cool completely before putting into pie tins otherwise the pastry will sweat
If making quick pies with puff pastry, cool then fill your tin or cast iron mini skillets, sealing around the edges if you want a cleaner look. then vent the top with a small slit in the pastry and cook according to pastry directions. Usually around 30 minutes at 400°