Coat a large saute pan, over medium heat, with olive oil and add the onions and celery. Season with salt and cook until the veggies start to become soft and are very aromatic. Add the garlic and cook for another 1 to 2 minutes. Add the sausage and cook until the sausage browns. Stir in the apples and apple cider and cook until the apples start to soften, about 3 to 4 minutes. Sprinkle in the sage leaves and the walnuts and turn off the heat.
Preheat the oven to 350 degrees F.
Add the diced bread and toss together. Pour in the chicken stock and knead with your hands until the bread is very moist, actually wet. Taste to check for seasoning and season with salt, if needed, (it does). Transfer to a large deep ovenproof dish (roughly 9 by 11 inches) and bake until it is hot all the way through and crusty on top.
Remove from the oven and serve.