Preheat the oven to 350°. Grease two 12-cup mini-muffin pans with cooking spray.
On a baking sheet, toss the bread with 2 tablespoons of the oil; season with salt and pepper. Bake for about 10 minutes, until toasted. Transfer the croutons to a bowl.
Meanwhile, in a large skillet, melt the butter in the remaining 2 tablespoons of oil. Add the onion and celery and cook over moderately high heat, stirring occasionally, until golden, about 5 minutes.
Add the sausage, garlic and sage and cook, stirring and breaking up the meat, until no trace of pink remains, 5 minutes.
Mix the sausage, apple, eggs and broth into the croutons; season with salt and pepper. Let stand for 5 minutes.
Pack the stuffing into the muffin cups and bake for 20 to 25 minutes, until golden. Transfer to a rack and let stand for 5 minutes. Loosen the muffins with a sharp paring knife and lift them out.