In a skillet over medium heat, cook the Italian sausage until browned and fully cooked. Transfer to a large mixing bowl.
In the same skillet, heat olive oil. Add onions and celery, and sauté until softened, about 5 minutes.
Add the apples and herbs, cooking for another 2-3 minutes. Season with salt and pepper.
Transfer the mixture to the bowl with sausage. Add bread cubes and toss to combine.
Pour in chicken broth and gently stir to moisten all ingredients.
Transfer stuffing mixture to a greased slow cooker. Dot with butter on top.
Cover and cook on low for 4 hours, or until heated through and slightly crisp around the edges.
Serve warm directly from the slow cooker.