Place two racks in the upper and lower thirds of your oven and preheat the oven to 300 degrees F. Spread the bread cubes out in a single layer on two large baking sheets. Bake for 15 minutes, until browned and crisp. Transfer to a large bowl.
Generously grease two 12-cup muffin tins (butter work best) or line with paper liners, then coat the liners with baking spray. Set aside. Increase the oven temperature to 350 degrees F.
Heat the olive oil in a large skillet over medium high. Once hot, add the onion, celery, salt, and pepper. Let cook 4 minutes, until the vegetables begin to soften. Remove the sausages from their casing and place in the skillet. Break the sausage into crumbles, turning and sautéing the meat as you go. Once the sausage is broken up, add the apples and garlic. Let cook until the sausage is browned, stirring occasionally, about 2 additional minutes. Place the sausage mixture in the bowl with the bread cubes.
In a separate bowl, stir together the chicken broth, milk, and eggs until combined. Pour over the cubes. Add the cranberries, sage, and rosemary, then stir gently with a large spoon, tossing the ingredients until they are well distributed and the bread is evenly moistened.
Spoon the mixture into the prepared muffin tins. Firmly press down on the tops of the bread cubes so that they touch the sides of the muffin tin (this adds a delicious crunch to the exterior and keeps the muffins from falling apart). Spoon more stuffing on top and press again. Bake the stuffing muffins at 350 degrees F for 25 to 28 minutes, until lightly browned and set in the center. Let cool in the pan for 10 minutes, then run a knife around the outsides to loosen them. Transfer to a wire rack. Serve warm.
To prep the Stuffing Muffins ahead: Toast the bread cubes (Step 1) and saute the apple-sausage mixture (Step 3). Store the bread cubes in an airtight container at room temperature and the apple-sausage mixture in the refrigerator. The day you plan to bake the muffins, stir together the rest of the ingredients and press into the muffin tins. You can then refrigerate the muffin tins, unbaked, for up to 4 hours, then bake as directed. If the stuffing comes out of the refrigerator cold, you may need to add a few minutes to the baking time.
To freeze the baked Stuffing Muffins: Allow the muffins to come to room temperature, tightly and individually wrap them in plastic, then freeze for up to 2 months. Let thaw overnight in the refrigerator, and when you are ready to bake, allow the muffins to come to room temperature. Place the room-temperature muffins (unwrapped) back into a lightly greased muffin tin, then reheat in a 350 degree F oven for 10-20 minutes, until warmed through.