If using sandwich bread, process 3 roughly torn slices white sandwich bread in a food processor until the consistency of fine coarse crumbs, about 20 seconds. Transfer to a large bowl. (If using panko breadcrumbs, just add to a large bowl.)

Melt 2 tablespoons unsalted butter in a medium skillet over medium heat. Add 1 finely diced medium yellow onion, 1 finely diced medium celery stalk, 1 small shredded apple, ½ teaspoon kosher salt, and ¼ teaspoon black pepper. Cook, stirring occasionally, until the onions and apples release their liquid and it evaporates, about 5 minutes. Stir in 1 tablespoon finely chopped fresh sage leaves and 1 teaspoon poultry spice, and cook until fragrant, about 30 seconds.

Transfer the onion mixture to the bowl with the breadcrumbs. Add 2 tablespoons low-sodium chicken broth and stir until combined. Let sit at room temperature to hydrate the breadcrumbs and cool slightly, about 20 minutes. Meanwhile, heat the oven to 400ºF. Line a baking sheet with parchment paper.

Add 1 pound raw breakfast sausage to the breadcrumb mixture and mix until evenly combined; do not overmix, or the meatballs will be tough. Scoop out the mixture in 2-tablespoon portions (about 24) and roll between damp hands until a smooth ball forms. Place on the baking sheet, spacing them evenly apart.

Roast until cooked through and browned all over, 25 to 30 minutes.