How to Make It
Spread bread pieces in a single layer on 3 rimmed baking sheets, and let stand, uncovered, at room temperature 8 hours or overnight to dry out.
Preheat oven to 375°F. Heat 2 tablespoons butter in a deep 13- to 14-inch skillet over medium until sizzling. Add celery and onion; cook, stirring often, until just softened, 10 to 13 minutes. Add thyme and rosemary; cook, stirring constantly, until fragrant, about 1 minute. Transfer celery mixture to a large bowl.
Wipe skillet clean, and add 1/4 cup butter. Heat over medium until sizzling. Stir in mushrooms. Increase heat to high; cook, undisturbed, until bottoms of mushrooms are browned, about 3 minutes. Stir mushrooms, and cook, stirring occasionally, until mushrooms are browned all over, about 20 minutes. Remove skillet from heat, and let stand 30 seconds. Add sherry to skillet, stirring and scraping up any browned bits from bottom of skillet. Return skillet to heat over medium; add parsley, salt, pepper, and remaining 1/4 cup butter; stir until butter is melted. Add mushroom mixture to celery mixture; add dried bread, and toss to combine.
Whisk together stock and eggs until thoroughly blended. Stirring constantly, slowly pour stock mixture into bread mixture. Continue stirring until most of stock is absorbed. Grease a 13- x 9-inch baking dish with butter; spoon bread mixture into baking dish, and cover tightly with aluminum foil.
Bake in preheated oven until warmed through, about 30 minutes. Uncover and cook until lightly browned, 15 to 20 minutes. Let stand 10 minutes before serving.