1. Preheat your oven to 400°F (200°C).

2. In a large skillet, melt the 2 tablespoons of butter over medium heat. Add the onion and garlic and sauté until fragrant, about 2-3 minutes.

3. Add the cubed chicken, potatoes, carrots, and celery to the skillet. Cook for 5-7 minutes, stirring occasionally, until the chicken is lightly browned.

4. Sprinkle the 2 tablespoons of all-purpose flour over the chicken and vegetables. Stir to coat evenly.

5. Gradually pour in the chicken broth and milk, stirring constantly to prevent lumps. Bring the mixture to a simmer and cook for 5-10 minutes, or until the sauce has thickened.

6. Stir in the frozen peas, dried thyme, and dried parsley. Season with salt and black pepper to taste.

7. Transfer the chicken pot pie filling to a 9-inch baking dish.

8. In a medium bowl, whisk together the 2 cups of all-purpose flour, baking powder, baking soda, and 1/2 teaspoon of salt.

9. Cut in the 6 tablespoons of cold, cubed butter until the mixture resembles coarse crumbs. Stir in the 3/4 cup of buttermilk until just combined.

10. Drop the biscuit dough by large spoonfuls onto the top of the chicken pot pie filling, covering the entire surface.

11. Bake for 20-25 minutes, or until the biscuits are golden brown and the filling is bubbly.

12. Let the Chicken Pot Pie with Biscuit Topping cool for 5-10 minutes before serving.