How to Make It
Preheat oven to 300°F with racks in upper and lower thirds. Spread bread on 2 rimmed baking sheets and bake until dry and crisp (but hasn't taken on any color), 18 to 20 minutes, rotating baking sheets from top to bottom halfway through. Let cool for 30 minutes. (Cooled, toasted bread cubes can be stored in an airtight container at room temperature for up to 2 days.)
While bread cools, increase oven to 350°F. Grease a 13-by-9-inch baking dish with 1 tablespoon butter. Heat oil and 2 tablespoons butter in a large skillet over medium-high until butter melts. Add sage; cook, stirring often, until crisp, about 1 minute. Using a slotted spoon, transfer sage to a plate.
Add onions, celery, pepper, and 1½ teaspoons salt to same skillet; cook, stirring often, until mixture softens and just begins to turn golden, 6 to 8 minutes. Transfer to a large bowl; add bread, grated apple, and half of the crisped sage.
Whisk eggs in a medium bowl until lightly beaten; add broth, vinegar, and remaining 1 teaspoon salt and whisk to combine. Pour broth mixture over bread mixture; stir gently to combine. Let mixture stand, stirring occasionally, until liquid is absorbed, 5 to 10 minutes.
Transfer bread-apple mixture to prepared baking dish. Top evenly with remaining crisped sage and dot with remaining 1 tablespoon butter. Cover dish with foil; bake at 350°F until steaming and slightly puffed, about 30 minutes. Uncover; continue baking until golden brown, 30 to 40 minutes. Let cool slightly for 5 to 10 minutes before serving.
A well-buttered pan and plenty of aromatics make every bite extra delicious.