Preheat the oven to 350°. Spread the bread cubes in a large roasting pan and toast for about 15 minutes, stirring once or twice, until slightly dry. Transfer to a large bowl.
In a large, deep skillet, melt 4 tablespoons of the butter. Add the apples, shallots, celery and sage and cook over high heat, stirring occasionally, until the apples are lightly browned in spots, about 8 minutes. Add the sausage pieces and cook over high heat, breaking them up with a spoon, until they're browned and cooked through, about 5 minutes. Add 3 cups of the stock and boil until nearly evaporated, about 6 minutes. Season with salt and pepper.
Add the sausage mixture to the bread along with the remaining 3 cups of stock. Stir until evenly moistened. Transfer the dressing to a large, buttered baking dish and dot with the remaining 2 tablespoons of butter.
Cover the dressing with foil and bake for 30 minutes. Remove the foil and bake for 30 minutes longer, until the dressing is browned around the edges. Let rest for 15 minutes before serving.