Preheat the oven to 250°. Scatter the bread cubes on a baking sheet and bake, tossing occasionally, until dried but not browned, about 40 minutes. Let cool. Increase the oven temperature to 350°.
Working in batches, pulse the celery in a food processor until chopped but not minced.
In a large, deep skillet, melt the butter over medium-high heat. Add the onions and cook until translucent, about 10 minutes. Mix in the celery and mushrooms and cook over high heat, stirring frequently, for 10 minutes. Stir in the salt and poultry seasoning until thoroughly combined.
In a large bowl, toss the bread cubes with the beaten egg mixture. Add the vegetable mixture and toss until well combined.
Spoon the stuffing into 2 buttered 3-quart casserole dishes and bake, uncovered, for 40 minutes.