Special equipment: 3-quart baking dish
Preheat the oven to 325 degrees F. Grease a 3-quart baking dish with butter.
In a 3-quart skillet, melt 2 sticks of the butter over medium heat and add the onions and celery. Cook until the vegetables are golden brown and caramelized, 35 to 40 minutes, stirring occasionally.
Add the torn bread pieces into the onion and celery mixture, and continue to cook until the bread becomes brown and crispy, 12 to 15 minutes.
In a large saucepan over medium-high heat, add the potatoes and fill just to cover with cold, salted water. Bring to a boil and cook until tender, 15 to 20 minutes. Drain and return the potatoes to the pot.
Using a potato masher, mash the potatoes until smooth. Add the milk, egg, 1 stick of the butter and the onion and celery mixture to the potatoes, and stir together with a wooden spoon.
Spoon the potato filling into the prepared baking dish. Dot the top of the potato filling with the remaining 2 tablespoons butter. Bake until the top is bubbling and browned, 30 to 45 minutes.
A viewer or guest of the show, who may not be a professional cook, provided this recipe. It has not been tested for home use.