Peel potatoes and cut into 1/2-inch chunks. In a large saucepan of boiling water, cover and cook potatoes until tender, 15 minutes; drain and return to pot. Add milk, cheese, butter, salt and pepper; mash until smooth. Set aside.
Cut chicken into 1/2-inch chunks. In separate large saucepan, bring stock to a boil. Add chicken, mushrooms, onion, carrots, garlic, bay leaves, salt, pepper and nutmeg; reduce heat, cover and simmer until chicken is no longer pink inside, about 6 minutes. Stir in peas, mustard and lemon juice.
In a small bowl, whisk flour with 1/3 cup cold water; whisk into stock mixture and bring to a boil. Reduce heat and cook, stirring often, until thick enough to coat back of spoon, about 5 minutes.
Make-Ahead Meal: Ladle into four 2-cup oven proof or foil dishes. Spoon or pipe potato mixture over top. Let cool for 30 minutes. Refrigerate until cold. Cover with foil and over wrap with heavy-duty foil; freeze for up to 1 month.
Thaw in refrigerator; remove heavy-duty foil. Preheat oven to 400°F. Bake on rimmed baking sheet until knife inserted in center for 5 seconds feels hot when touched, about 25 minutes. Uncover and broil until golden, about 3 minutes.
Calories From Fat
% Daily Value*
Total Fat 0g
Saturated Fat 0g
Trans Fat 0g
Dietary Fibers 0g
* Percent Daily Values are based on a 2,000 calorie diet.
* Nutrition information is estimated based on the ingredients and cooking instructions as described in each recipe and is intended to be used for informational purposes only. Please note that nutrition details may vary based on methods of preparation, origin and freshness of ingredients used.