01 - Preheat the oven to 400°F.
02 - In a large skillet over medium heat, melt 3 tablespoons butter. Add onion, carrots, and celery. Sauté for 5 minutes until softened.
03 - Stir in flour and cook for 1 minute. Gradually whisk in chicken broth and milk.
04 - Add salt, pepper, thyme, and garlic powder. Bring to a simmer, stirring constantly until thickened, about 3-5 minutes.
05 - Stir in diced chicken and peas. Simmer for 2 more minutes, then transfer mixture to a 9x13-inch baking dish.
06 - In a large bowl, whisk together flour, baking powder, sugar, and salt. Cut in cold butter until mixture resembles coarse crumbs.
07 - Stir in cheddar cheese and parsley. Add milk and mix just until combined. Do not overmix.
08 - Drop large spoonfuls of biscuit dough evenly over the chicken filling.
09 - Bake for 25-30 minutes, until biscuits are golden and cooked through.
10 - Mix melted butter with garlic powder and parsley, then brush over hot biscuits.
11 - Let cool for 5 minutes before serving.