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Preheat the oven to 400°. Slice the bread 1 inch thick. Using your fingers, pull the bread into coarse shreds. Spread the shreds of bread on 2 large, rimmed baking sheets. Drizzle with the olive oil and season with salt and pepper. Toss the bread to coat thoroughly with the olive oil. Bake the bread on the upper and lower oven racks for about 20 minutes, shifting the baking sheets between racks halfway through, or until the shreds are golden brown and lightly crisp. Let the toasted bread cool.
Melt the butter in a medium skillet. Add the sliced garlic and cook over moderately low heat, stirring once or twice, until fragrant and golden brown, about 3 minutes. Add the sliced scallions and sliced celery ribs and cook over moderately low heat, stirring once or twice, until the scallions are softened but the sliced celery is still slightly crisp, about 6 minutes.
Just before serving the bread salad, transfer the toasted bread to a very large bowl. Add the toasted pumpkin seeds and the chopped celery leaves. Reheat the scallion mixture over moderately high heat until sizzling. Stir in the sherry and simmer for 30 seconds. Scrape the scallion mixture over the toasted bread in the bowl and toss until thoroughly combined. Season with salt and pepper and serve at once.
The bread salad can be prepared through Step 2 early in the day and stored at room temperature. Reheat the scallion mixture and toss with the toasted bread just before serving.