The added step of drying the bread in the oven (rather than just using stale bread) allows the cubes to soak up even more of the fruit-infused chicken stock, as well as the flavors from the greens and sausage.
Preheat oven to 250 degrees. Bake bread on a baking sheet in a single layer until dried but not browned, about 15 minutes. Remove bread, and let cool. Raise oven temperature to 350 degrees.
Bring stock to a boil in a medium saucepan. Remove from heat. Add prunes, and let soak for 20 minutes.
Meanwhile, heat 2 tablespoons oil in a large high-sided skillet over medium-high heat. Working in batches, cook greens until tender, about 10 minutes, and transfer to a plate. Let cool. Wipe out skillet.
Heat remaining tablespoon oil in skillet over medium heat. Brown sausage, stirring occasionally, until just cooked through and no longer pink, about 10 minutes. Transfer to a large bowl using a slotted spoon. Add more oil to skillet if needed, and cook celery, carrots, and onions until tender, about 6 minutes. Season with salt and pepper.
Drain prunes, reserving poaching liquid. Add prunes, bread, greens, and vegetable mixture to bowl with sausage. Gradually add reserved poaching liquid (about 2 1/2 cups), stirring to combine. Stir in parsley and thyme. Season with salt and pepper.
Divide stuffing between two 9-by-13-inch baking dishes. Dot tops with butter. Bake until browned, about 45 minutes.
Storage: Stuffing can be refrigerated for up to 3 days. Reheat, covered, in a low-temperature oven.