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.
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.