Preheat oven to 350 degrees F (175 degrees C). Grease two 12-cup mini muffin tins with cooking spray.
Toss bread cubes with 2 tablespoons olive oil on a baking sheet; season with salt and pepper.
Bake bread cubes in the preheated oven until toasted, about 10 minutes. Transfer to a large bowl.
Melt butter and 2 tablespoons olive oil in a large skillet over medium-high heat. Add onion and celery; cook and stir until golden, about 5 minutes. Stir sausage, garlic, and sage into onion mixture and cook, breaking up the meat until no trace of pink remains, about 5 minutes.
Mix sausage mixture, eggs, apple, and chicken broth into bowl of toasted bread cubes. Season with salt and pepper. Let stand until flavors absorb, about 5 minutes.
Scoop stuffing mixture into muffin cups and bake in the preheated oven until golden, 20 to 25 minutes.
Transfer muffin tins to a rack to cool, about 5 minutes. Loosen the muffins with a sharp paring knife to lift them out.