Cut the challah into 1-inch cubes. Arrange the cubes on a parchment-lined sheet pan on the counter to get stale, at least overnight and preferably for 2 days.

Heat the oven to 350 F. In a large heavy skillet over medium heat, melt 3 ounces of the butter. Cook the onions, stirring, for about 5 minutes. until wilted. Add the celery, mushrooms, thyme, rosemary, and parsley and cook, stirring, until fragrant and warmed through, about 5 minutes more.

Meanwhile, in the microwave or in a small skillet over medium heat, melt the remaining 4 ounces of the butter.

Transfer the vegetables to a large bowl. Add the stock, bread, and melted butter; season with salt and pepper.

Press the stuffing into a large buttered baking dish. Cover with buttered parchment, then cover with foil. At this point, the stuffing can be held for several hours, but should be at room temperature before baking.

Bake, uncovering the pan during the last 10 to 15 minutes, for 45 to 55 minutes total, until the surface is crisp and the bread is golden brown.