Cut your loaf of sourdough bread into 1-inch cubes. Spread them on a rimmed baking sheet and let sit overnight to become slightly stale, or bake at 300°F for 10–15 minutes until dry but not browned.
In a large skillet over medium-high heat, melt 1 stick of butter. Stir in the celery, onions, and garlic. Season with kosher salt, black pepper, and poultry seasoning. Cook for 5–6 minutes, until the onions turn translucent and begin to soften.
In a large mixing bowl, add the dried bread cubes. Pour the vegetable mixture over the top and toss until evenly coated.
In a small bowl, whisk the egg with chicken or turkey broth. Slowly pour this over the bread mixture, stirring gently. Add just enough broth so the bread is moist but not soggy.
Brush a casserole dish with ¼ stick of butter. Add the stuffing mixture, dot the top with the remaining butter, and bake at 400°F for 15–20 minutes, or until golden brown and crisp on top.
Serve hot from the oven or at room temperature alongside your favorite holiday dishes.