Step 1

To prepare topping: Place potatoes in a large saucepan and add cold water to cover by 2 inches. Bring to a boil and cook until tender, 12 to 14 minutes. Drain and return the potatoes to the pan. Add milk, 2 tablespoons oil and 1/2 teaspoon salt; mash.

Step 2

Preheat oven to 400degrees F. Coat an 8-inch-square pan or similar 2-quart baking dish with cooking spray.

Step 3

To prepare filling: Heat oil in a large nonstick skillet over medium-high heat. Add mushrooms, onion, thyme and 1/4 teaspoon salt; cook, stirring frequently, until the mushrooms are starting to brown, 7 to 9 minutes. Pour in sherry and cook, stirring, until almost evaporated, 1 to 2 minutes. Whisk broth (or stock) and flour in a bowl. Add to the pan and cook, stirring, until thickened, 1 to 2 minutes. Stir in chicken, peas and carrots, the remaining 1/4 teaspoon salt and pepper; cook, stirring, for 1 minute. Transfer to the prepared baking dish. Spread the mashed potatoes on top.

Step 4

Place the baking dish on a baking sheet. Bake until the potatoes are browned in spots and the filling is bubbling, 30 to 35 minutes.