Preheat oven to 450 degree. In a mixing bowl, whisk together flour, baking powder, salt, pepper and garlic powder.
Add the butter and cut in with a fork until the mixture resembles coarse crumbs. Stir in Parmesan cheese.
Pour in half and half and mix just until combine. Crumble and drop mixture into small 1-2-inch pieces onto a greased baking dish (I use a jelly roll pan).
Bake in preheated oven for 10 minutes, until golden brown. Remove from oven and set aside.
For the filling
In a saucepan, combine chicken, frozen vegetables, and celery. Add water to cover the contents and then boil for 10-15 minutes or until chicken is cooked through.
Remove from heat and strain the water into a container and reserve for later. Chop chicken into small pieces.
In the saucepan over medium heat, cook onions in butter until soft and translucent.
Stir in flour, salt, pepper, garlic powder, bullion paste and celery seed. Slowly stir in reserved water and milk.
Simmer over medium-low heat until thickened. Add chicken and vegetables. Taste and season with more salt, pepper, or garlic powder if necessary.
Pour mixture into pie dish. Place baked crumble pieces on top. Bake at 400 degrees F for 12-14 minutes.