Preheat the oven to 375ºF. Spray a 9x13-inch casserole dish with nonstick cooking spray. Set aside.
In a skillet set over medium/high heat, partially brown the Italian sausage. Cook until there is a little pink left, about 3-4 minutes. Transfer the partially cooked sausage into the casserole dish.
In the same skillet, add 1.5 tablespoons of olive oil. Add both kinds of potatoes, sweet potatoes, Yukon potatoes, 1/2 teaspoon garlic powder, and 1/4 teaspoon salt. Cook just long enough to partially cook through, about 3-5 minutes. Transfer the potatoes into the casserole dish.
In the same skillet, caramelize the onion. Heat 2 more tablespoons of olive oil. Add in the thinly sliced onion and cook over low/medium heat until the onions are translucent and caramelized, about 7-10 minutes. Transfer the caramelized onions into the casserole dish.
To the casserole dish, add the apple, fresh herbs, remaining garlic powder and salt, pepper, and bread cubes. Mix everything together using a spatula (or your hands).
In large glass measuring cup (or bowl), mix together the beef broth and Worcestershire sauce. Pour the liquid over the stuffing mix in the casserole dish and mix again.
Place thin pads of butter on top of the sausage stuffing. Bake, uncovered, until the top is golden brown and firm to the touch, about 35-45 minutes. If you notice the top is browning too quickly, tent a piece of foil over the top of the casserole dish halfway through baking.