1Heat the oven to 350°F and arrange a rack in the middle.
2Remove the crust from the bread. Cut the bread into 1/2-inch cubes and place on a baking sheet. Spread into an even layer and bake until light golden brown and dry to the touch, stirring halfway through, about 10 minutes total. Set aside to cool.
3Meanwhile, heat the vermouth in a medium saucepan over medium heat until simmering. Add the prunes and simmer, stirring occasionally, until the vermouth has been absorbed, about 2 minutes. Transfer to a large bowl and set aside.
4Heat a large frying pan over medium-high heat until hot, about 2 minutes. Add the sausage and, using two forks, break it into very small pieces, adjusting the heat if the sausage is browning too quickly. Cook, stirring occasionally, until no longer pink, about 4 minutes. Using a slotted spoon, transfer the sausage to the bowl with the prunes.
5Set the pan back over medium-high heat and add the butter. Once the butter foams, add the onions, sage, salt, and pepper and cook until the onions are softened, stirring occasionally and scraping up the browned bits from the bottom of the pan, about 6 minutes (adjust the heat if the onions are browning too quickly). Add the celery and cook, stirring occasionally, until starting to soften, about 6 minutes. Add the apples and cook, stirring occasionally, until starting to soften, about 3 minutes. Transfer the mixture to the bowl with the prunes and sausage. Add the toasted bread cubes and the eggs and stir to combine. Taste and season with additional salt, pepper, and sage as desired.
6If using to stuff a turkey or goose, do not bake and cool to room temperature before stuffing the bird. If baking separately, transfer to a 13-by-9-inch baking dish and bake uncovered until golden brown on top, about 35 to 40 minutes.