Lightly grease a 6-quart slow cooker. Preheat the oven to 350°F. Spread bread cubes in a single layer on two large, rimmed baking sheets. Bake for 12 minutes, then stir cubes and sprinkle nuts over the top. Continue baking until nuts are toasted and bread cubes are dry, about 5-8 minutes. Watch the nuts carefully towards the end so they do not burn. Remove from the oven and place immediately in the prepared slow cooker.
Meanwhile, heat oil and butter in a large nonstick skillet over medium. Once the oil is hot and the butter is melted, add onions, salt, and pepper. Cook until the onions have softened, about 5 minutes. Add celery and continue cooking until the celery is soft and the onions are golden, about 5-8 minutes. Stir in sage and thyme and let cook 1 minute. Add to the slow cooker.
Add diced apple and cranberries to the slow cooker, stirring gently to evenly distribute. In a separate bowl, whisk eggs and broth until well combined. Pour over bread cube mixture, then fold gently to combine.
Lay a clean kitchen towel over the top of the slow cooker so it hangs over all sides, to absorb excess moisture. Cover with the lid, letting the towel poke out of the sides. Cook on low for 3-4 hours, until lightly crisp around the edges and set in the center. Serve warm.