If making the stuffing immediately, preheat oven to 375 degrees.
Heat a large skillet over medium heat. Add the Italian sausage and use a spoon to break it up while cooking. Cook for about 5-6 minutes, until cooked through. Remove the sausage from the skillet and set aside.
Return the skillet to the stove and add in butter, celery, onion, apple, sage, thyme, and salt. Cook until all of the vegetables are softened, about 9-10 minutes.
Use a serrated knife to cut the focaccia into 1-inch cubes. Place in a very large mixing bowl. Add in the cooked sausage and veggies. Toss well to combine. (If making ahead, refrigerate this mixture and proceed to next step when ready to bake.) Add the beaten egg and chicken stock to the stuffing and toss again.
Pour the stuffing into a 9 x 13 casserole dish. Cover with foil and bake for 30 minutes. Uncover and bake for another 10 minutes until top is lightly browned and internal temperature reaches 165 degrees.
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Notes
What makes this focaccia stuffing special is the bread. However, you can still make it without focaccia. Just use day old Italian or French bread cut into cubes. Add an extra teaspoon of dried rosemary when cooking the veggies.