Baked apples are usually stuffed with nuts and sticky-sweet brown sugar or maple syrup. But apples are also delicious with pork, so we’ve gone savory here and stuffed them with toasted walnuts, grilled cipollini onions, sweet sausage, and sage. On the grill, the apples pick up a hint of smoke, which enhances the taste of the sausage. There’s a bit of chopping and mixing, so bring a cutting board to the grill for this one.
Light a grill for indirect medium heat, about 350 degrees.
Cut the onions in half through their equators, and then toss with the unpeeled garlic and oil in a bowl. Brush the grill grate and coat with oil. Grill the onions, cut side down, and the unpeeled garlic directly over the heat until the onions are tender and the garlic is charred in spots, about 5 minutes, turning once or twice. Squeeze the garlic from its skins, and then chop along with the onions.
Put a large cast-iron skillet or heavy-duty roasting pan directly over the heat. Add the walnuts and toast until fragrant, about 5 minutes, shaking occasionally. Remove the walnuts from the pan and chop.
Add the sausage to the pan and cook until lightly browned all over, 5-8 minutes, stirring occasionally. Remove and set aside. Add the celery to the drippings in the pan and cook for 4 minutes, stirring now and then. Add the onions, garlic, sausage, and the poultry rub and cook for 1 minute. Scrape the stuffing into a bowl.
Cut the apples in half crosswise and remove the core from each half with a melon baller. Put the apples, cut side up, in the skillet or in a roasting pan. Fill the center of each half with the sausage mixture. Mix together the apple cider, liqueur, vinegar, and sage and pour around and over the apples. Cover the pan with foil and put the pan on the grill away from the heat. Cover the grill and cook for 20 minutes. Remove the foil and cook until the apples are tender, 15-20 minutes more.
Let cool for 10 minutes before serving. Drizzle the apples with the cider syrup from the pan and serve.