Measure 10 cups (approximately 400 g) of dried bread cubes into a large mixing bowl. Core and dice 2 medium apples into 1/2-inch (1.3 cm) pieces. Finely dice 1 medium yellow onion and 3 medium celery stalks. Keep the prepared ingredients ready while you heat the skillet. The bread should feel dry and firm, and the fruit and vegetables should be cut into fairly even pieces.

Place a 12-inch (30 cm) skillet over medium heat. Add 1 pound (454 g) of bulk mild pork breakfast sausage. Cook for approximately 8 to 10 minutes, breaking the meat into small crumbles with a wooden spoon. Continue cooking until the sausage is browned, no pink remains, and the center of the meat reaches 160°F (71°C). Transfer the sausage to the bowl with the dried bread cubes, leaving a thin layer of drippings in the skillet.

Add 3 tablespoons (43 g) of unsalted butter to the same skillet over medium heat. Once melted, add the diced onion and celery. Cook for approximately 5 minutes, stirring frequently, until the onion looks translucent and the celery begins to soften. Stir in 2 teaspoons (2 g) of dried rubbed sage, 1 teaspoon (1 g) of dried thyme, and 1/2 teaspoon (1 g) of freshly ground black pepper during the final 30 seconds. Remove from the heat when the herbs smell fragrant.

Add the warm onion and celery mixture to the bowl containing the sausage and dried bread cubes. Add the 2 diced apples and gently toss with a large spoon until everything is evenly distributed. Pour in 1 1/2 cups (355 ml) of low-sodium chicken broth, distributing it across the bread rather than pouring it into one spot. Fold gently until the bread looks evenly moistened but still holds its shape. Avoid crushing the cubes or mixing them into a paste.

Rub the remaining 1 tablespoon (14 g) of unsalted butter over the bottom and sides of a 6-quart (5.7 L) slow cooker. Spoon the stuffing mixture into the cooker, spreading it into an even layer without packing it down tightly. The surface should look lightly moist, with distinct bread cubes and visible pieces of sausage and apple.

Cover the slow cooker with its lid and cook on LOW for approximately 4 hours. Keep the lid closed during most of the cooking time to maintain a steady temperature. The stuffing is ready when the apples and celery are tender, the bread is soft in the center, and the edges are lightly golden. Check several spots with an instant-read thermometer to confirm the stuffing reaches at least 165°F (74°C).

Once the stuffing reaches 165°F (74°C), turn off the slow cooker and remove the lid. Gently lift a small portion from the center with a serving spoon to check the texture. The bread should be moist but not swimming in liquid, and the sausage should be evenly distributed. If the surface looks slightly damp, leave the lid off for 5 minutes before serving. Avoid stirring repeatedly, which can break down the softened bread.

Fluff the cooked stuffing gently with a large serving spoon, lifting from the edges toward the center. Serve it directly from the slow cooker or transfer it to a serving dish. You should see tender golden bread cubes, browned sausage, and soft apple pieces throughout. Serve while the stuffing is hot, with an internal temperature of at least 165°F (74°C) at the end of cooking.