Boil the sweet potato chunks in salted water 15 to 18 minutes, until a fork slides through with no resistance. Drain well and let them steam dry for 2 minutes.

Mash with the butter, warmed milk, salt and pepper until thick and spreadable, closer to frosting than to a puree. Set aside.

Melt the 4 tablespoons of butter in a large skillet over medium heat. Cook the onion, carrot and celery 7 to 8 minutes until softened, then add the garlic, thyme and sage and cook 1 minute more.

Sprinkle the flour over the vegetables and stir constantly for 2 minutes, until it smells nutty rather than pasty.

Add the chicken stock gradually, whisking between additions, then simmer 4 to 5 minutes until the sauce coats the back of a spoon and holds a line when you draw through it. Stir in the ½ cup milk and take the pan off the heat.

Fold in the shredded chicken and frozen peas, season with the salt and pepper, then scrape everything into a 9 by 13 inch baking dish or large oval baker and level the top while it is still hot.

Drop the sweet potato mash over the hot filling in 8 or 9 separate dollops, then gently join them into one even layer working from the edges inward. Rake ridges across the surface with a fork once the layer is even.

Bake at 375F for 25 to 30 minutes, until the filling bubbles at the edges of the dish.

Run the dish under the broiler for 2 to 3 minutes, watching constantly, until the ridges are deep and burnished. Rest 10 minutes and finish with the chopped parsley.