Preparation Preheat oven to 400°F with rack in lower third. Butter a 11/2-qt shallow baking dish or gratin dish. Cook onion and celery in 6 tablespoon butter with 1/2 teaspoon salt and 1/4 teaspoon pepper in a large heavy skillet over medium heat, stirring occasionally, until softened, 8 to 10 minutes. Transfer to a bowl and toss with bread cubes, celery leaves, and sage, then cool 5 minutes. Whisk together stock and egg (if stock is hot, gradually whisk into egg), then toss with bread mixture until absorbed. Transfer to baking dish and dot top with remaining tablespoon butter. Bake, covered with foil, 30 minutes, then uncover and bake until top is golden, about 10 minutes more. Cooks' note:Stuffing can be assembled (but not baked) 1 day ahead and chilled (covered once cool). Bring to room temperature before baking.