Preheat oven to 350°F (175°C).

In a large skillet, melt butter over medium heat. Add the chopped onion and celery; cook until softened.

Stir in flour, cooking for 1 minute.

Gradually whisk in chicken broth and milk, bringing to a simmer until thickened.

Stir in kosher salt, black pepper, celery seed, cooked chicken, and frozen mixed vegetables. Mix well.

In a separate bowl, combine flour, baking powder, kosher salt, and sugar. Cut in grated butter until mixture resembles coarse crumbs.

Stir in milk until just combined.

Pour the chicken mixture into a baking dish and top with the biscuit mixture.

Bake for 18-20 minutes or until the top is golden brown.