Lightly coat a 6- or 7-quart slow cooker with nonstick spray. Position oven racks in the upper and lower thirds of your oven and preheat your oven to 350 degrees F. Spread the bread cubes in a single layer on two large, rimmed baking sheets.
Bake for 12 minutes, then stir the bread cubes and sprinkle the nuts over the top. Continue baking until the nuts are toasted and crisp and the bread cubes are dry, about 5 to 8 additional minutes. Watch the nuts very carefully towards the end so that they do not burn. Remove from the oven and place immediately in the prepared slow cooker.
Meanwhile, heat the olive oil and butter in a large nonstick skillet over medium. Once the oil is hot and the butter is melted, add the onions, salt, and pepper. Cook until the onions begin to soften, about 5 minutes. Add the celery and continue cooking until the celery is soft and the onions are golden, about 5 to 8 additional minutes.
Stir in the sage and thyme and let cook 1 additional minute. Add to the slow cooker with the bread cubes.
Add the diced apple and cranberries to the slow cooker, then with a large spoon, very gently stir to evenly distribute. In a separate bowl, whisk the eggs and broth until well combined. Pour over the bread-cube mixture, then fold gently to combine. Let the mixture sit a few minutes, then fold again so that the bread is evenly moistened.
Lay a clean kitchen towel or a double layer of paper towels over the top of the slow cooker so that you have overhang on all sides. This will help absorb moisture. Cover, letting the paper towel poke out the sides of the lid, and cook on low for 3 to 4 hours, until lightly crisp around the edges and set in the center. If desired, sprinkle with fresh parsley. Serve warm.
Notes
TO MAKE AHEAD: You can individually prep all the stuffing components (bread and nuts, vegetables, egg-and-broth mixture), then store them separately, keeping the bread/nuts at room temperature and the rest in the refrigerator. Combine just before cooking.
TO STORE: Refrigerate stuffing in an airtight storage container for up to 4 days.
TO REHEAT: Gently rewarm leftovers in a baking dish in the oven at 350 degrees F. You can also keep stuffing warm in a slow cooker on the "warm" setting.
TO FREEZE: Freeze stuffing in an airtight freezer-safe storage container for up to 3 months. Let thaw overnight in the refrigerator before reheating.
I do not recommend cooking this recipe on high, as it may not cook all the way through before the edges begin to burn.