Preheat the oven to 350F and grease a 9x9 baking dish.
Make the filling:
Heat a large pan over medium-high heat. Once hot, add the oil and spread it around the pan. Add the onions and celery, and cook them while stirring frequently for 3 to 5 minutes until the onions have softened and are translucent. Season it with the garlic powder, paprika, and poultry seasoning, and stir.
To the pan, add the mixed vegetables and cooked and chopped chicken pieces. Stir to combine. Pour the chicken stock into the pan and stir to cook and heat everything through for about 5 minutes. Add the milk, stir, and reduce the heat to medium-low, letting it cook while it comes up to a simmer.
Make a slurry by mixing the cornstarch and water in a small dish. Add it to the simmering filling in the pan and combine. Cover the pan with a lid and continue simmering the mixture for 3 minutes and turn off the heat.
Assemble:
Transfer the warm filling from the pot to the prepared baking dish. Open the can of biscuits and place the biscuits on top of the baking dish.
Bake:
Bake in the preheated oven for 15 minutes, until the tops of the biscuits are golden brown. Check the baking time in the packaging.
When the biscuits are ready, remove the baking dish from the oven and serve warm.