Preheat oven to 350 degrees F (175 degrees C). Coat a 9×13 baking dish with cooking spray.
Bring a large pot of salted water to a boil. Cook potatoes in boiling water until tender, about 15 minutes. Drain, reserving some of the cooking liquid. Mash potatoes with a little of the cooking liquid. Set aside.
In a large skillet, cook ground beef until brown over medium-high heat. Drain fat from pan. Stir in mushrooms, mixed vegetables, mushroom soup, celery soup, and salt and pepper; heat through. Pour into prepared baking dish, cover with mashed potatoes, and dot with butter.
Bake in preheated oven for 30 minutes, or until potatoes are golden and beef and vegetable mixture is hot and bubbly.
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