Preheat the oven to 400°F. Lightly grease a 9-by-13-inch baking dish.
Melt 4 tablespoons of butter in a large skillet over medium heat. Add the onion, celery, and bell pepper. Cook for 5 to 6 minutes, stirring occasionally, until beginning to soften.
Add the garlic and cook for 30 seconds, just until fragrant.
Sprinkle 1/3 cup flour over the vegetables. Stir constantly and cook for 1 to 2 minutes.
Add AB’s SPG, smoked paprika, thyme, oregano, and cayenne. Stir for approximately 30 seconds.
Slowly pour in the chicken broth while whisking or stirring constantly. Add the heavy cream and Worcestershire sauce.
Bring the mixture to a gentle simmer. Cook for 3 to 5 minutes, stirring frequently, until the gravy lightly coats the back of a spoon.
Fold in the cooked chicken and frozen mixed vegetables. Simmer for another 2 to 3 minutes. Taste and adjust the seasoning if needed.
Transfer the hot filling to the prepared baking dish and spread it evenly.
To make the biscuit topping, whisk together 2 cups flour, baking powder, garlic powder, onion powder, and salt in a large bowl.
Add the cold grated butter and toss it through the flour. Fold in the shredded cheddar cheese.
Pour in the cold buttermilk and stir just until a shaggy dough forms. Do not overmix.
Drop approximately 12 equal portions of biscuit dough over the hot filling, leaving a little space between them.
Place the casserole dish on a rimmed baking sheet. Bake uncovered for 25 to 30 minutes, or until the filling is bubbling and the center biscuits are cooked through and golden.
Mix the melted butter, garlic powder, and parsley. Brush it over the hot biscuits.
Rest the cobbler for 10 minutes before serving.