Preheat your oven to 375°F (190°C). If your sourdough bread is fresh, spread the cubes on a baking sheet and bake for 10-15 minutes until lightly toasted and dry. Set aside.
In a large Dutch oven or oven-safe skillet, cook the Italian sausage over medium-high heat, breaking it up with a spoon, until browned and cooked through, about 5-7 minutes. Do not drain the fat.
Add the butter to the skillet. Once melted, add the chopped onion and celery. Cook, stirring occasionally, until softened, about 5 minutes.
Stir in the diced apples and continue to cook for another 3-4 minutes until they begin to soften. Add the minced garlic and cook for 1 minute more until fragrant.
Pour in 2.5 cups (600 ml) of the chicken stock. Bring the mixture to a simmer and let it cook for 3-4 minutes to allow the flavors to combine.
Remove the skillet from the heat. Add the dried sourdough cubes, chopped sage, salt, and pepper. Gently fold everything together until the bread starts to absorb the liquid.
In a small bowl, whisk the egg with the remaining 1/2 cup (120 ml) of chicken stock. Pour this over the stuffing mixture and fold gently to combine everything evenly.
If your skillet is not oven-safe, transfer the stuffing to a greased 9x13-inch baking dish. Bake, uncovered, for 30 minutes.
Increase the oven temperature to 400°F (200°C) and bake for an additional 5-10 minutes, or until the top is golden brown and crisp. Confirm the stuffing has reached an internal temperature of 160°F (71°C) since it contains ground meat. Let rest for a few minutes before serving.