Preheat oven to 350°F. Coat a 9"x13" baking dish with cooking spray. Cook potatoes in boiling water until tender, about 15 minutes. Drain, reserving some of the cooking liquid. Peel & quarter the potatoes, then mash with a little of the cooking liquid. Set aside.
In a large skillet, cook the meat over medium-high heat. Drain fat from pan. If using leftover meat, heat & add to the skillet. Stir in mushrooms, vegetables, mushroom & celery soups, salt, & pepper; heat through. Pour into prepared baking dish; cover with mashed potatoes & dot with butter.
Bake in preheated oven for 30 minutes or until potatoes are golden & meat & vegetable mixture is hot & bubbly.