Preheat oven to 225F. Cut bread into ½-inch cubes and lay on 2 unlined baking sheets. Bake in oven at 225F for 30 minutes. This dries out the bread without having to wait for it to go stale.
Once your bread is out of the oven, increase the oven heat to 350F and grease a 9x13 casserole dish. Heat a large pot over medium-low and melt the stick of butter.
Pour the onions and celery into the pot with the melted butter and toss so they are coated. Allow the onions and celery to cook down until they are super soft, about 10 minutes.
Pour salt, pepper, and parsley into the pot and toss to combine. Turn off the heat and pour the cubed bread into the pot. Toss until all of the bread is coated in the butter and veggie mixture.
Pour the broth into the pot and mix again. Whisk the egg and pour into the pot, tossing immediately so it's evenly spread throughout.
Pour mixture from the pot into the greased casserole dish. Cover with foil and bake covered for 45 minutes. Uncover and bake for another 20-25 minutes to crisp the top. Garnish with a bit of fresh parsley and serve warm.
Notes
Food safety: Because this stuffing contains egg, bake until the center reaches 165°F on an instant-read thermometer.
Adjusting moisture: This is meant to be a moist and soft stuffing with a crisp top. For drier, crispier interior, use 1/2 cup less broth.
Butter swap: If using unsalted butter, add an extra 1/2 tsp kosher salt.
Doubling: To double the recipe, bake in two 9×13 dishes rather than one deeper dish so it cooks evenly and gets a crispy top.
Bread size: Cut the bread into small ½-inch cubes. Smaller pieces hold together better and absorb the butter and broth more evenly. Try to make the bread pieces even in size so the dish bakes up evenly.
Vegetarian: Use vegetable broth instead of chicken or turkey broth. To make the dish vegan, use a vegan butter substitute and vegetable broth.