In a large cast-iron skillet or Dutch oven, heat the olive oil and butter on medium-high heat. When the oil is hot, add the onion and celery and stir well. Cook for five minutes, stirring frequently.
Add the apples and stir to combine. Cook for 10 minutes, stirring frequently.
Add the garlic and sausage and stir to combine. Cook for 5 minutes, stirring frequently.
Add the toasted walnuts, herbs, and salt and pepper and stir to combine and cook for two minutes.
Stir in the breadcrumbs and pour the vegetable broth evenly over the top. Stir the broth in until the bread is covered in the broth. You can add more if there are any dry crumbs, but you don’t want them swimming in liquid or it will get mushy.
Put the pan in the oven and roast for 15 minutes, or until the top is browned and crispy.