Preheat the oven to 400 degrees F. Melt 2 tablespoons of butter in a large oven-safe skillet over medium heat. Add the chicken pieces and cook until lightly browned on all sides.
Add the onion, carrots, and celery to the skillet. Cook for 5 minutes until the vegetables begin to soften.
Sprinkle the 1/4 cup flour and thyme over the chicken and vegetables. Stir for 1 minute, then slowly pour in the chicken broth while stirring to avoid lumps.
Add the heavy cream, frozen peas, salt, and pepper. Simmer for 3 minutes until the sauce thickens slightly. Remove the skillet from the heat.
In a mixing bowl, whisk together the 1 1/2 cups flour, baking powder, and 1/2 teaspoon salt. Cut in the cold cubed butter until the mixture resembles coarse crumbs. Stir in the milk just until a soft dough forms.
Drop spoonfuls of the biscuit dough over the filling in the skillet. Bake for 20 to 25 minutes until the biscuits are golden and the filling is bubbling.