Preheat your oven to 375°F.
In a large skillet over medium heat, add the Italian sausage. Use a spoon to break it up as it cooks. Continue cooking for about 5-6 minutes until it’s browned and cooked through. Remove the sausage from the skillet and set it aside, leaving the rendered fat behind.
Return the skillet to the stove and add ¼ cup of butter. Once melted, add the diced celery, onion, and apple, along with the fresh sage, dried thyme, and salt. Cook, stirring occasionally, for 9-10 minutes until the vegetables have softened.
Place the cubed focaccia in a very large mixing bowl. Add the cooked sausage and the sautéed vegetable mixture to the bowl. Toss everything together until it’s well combined.
Add the 1 beaten large egg and 3 cups of broth to the focaccia mixture. Toss gently but thoroughly to ensure the liquid is evenly absorbed by the bread cubes.
Pour the stuffing mixture into a 9×13 inch casserole dish and spread it into an even layer. Cover the dish tightly with foil. Bake in your preheated oven for 30 minutes.
After 30 minutes, carefully remove the foil. Return the dish to the oven and bake for another 10 minutes, or until the top is lightly browned and crisp and the center registers 165°F.