Heat the oven to 425°F and melt 2 tablespoons butter in a 10-inch oven-safe skillet.
Cook the onion until soft, whisk in 3 tablespoons flour for 1 minute and gradually add the broth and 1/2 cup milk.
Simmer until thick, then stir in 1/2 teaspoon garlic powder, 1/2 teaspoon salt, black pepper, cooked chicken and frozen vegetables.
Whisk 2 cups flour, baking powder, Parmesan, remaining garlic powder and remaining salt in a bowl.
Melt the remaining butter and stir it with the remaining milk, then fold the liquid into the dry ingredients.
Drop 8 mounds of biscuit dough over the hot filling, leaving gaps between them.
Bake until the biscuits are browned and cooked through, 22 to 26 minutes.