Preheat oven to 375°F.

In a large skillet, melt 2 tablespoons of butter over medium heat. Add onions, carrots, and celery, and cook until softened, about 5 minutes.

Stir in flour to coat the vegetables. Gradually pour in chicken broth and heavy cream, stirring constantly until the mixture thickens, about 5 minutes.

Add the cooked chicken and frozen peas. Stir until well combined and heated through. Season with salt and pepper to taste.

Transfer the filling to a greased 9x13 inch baking dish.

In a mixing bowl, combine flour, baking powder, and salt. Add cold butter and use a pastry cutter or your fingers to blend until the mixture resembles coarse crumbs.

Pour in milk and stir just until the dough comes together.

Drop spoonfuls of biscuit dough over the chicken mixture.

Bake in the preheated oven for 25-30 minutes, or until the biscuits are golden brown and the filling is bubbly.

Let cool for 10 minutes before serving.