Warm 1 tablespoon of oil or butter in a large, heavy-bottomed skillet over medium heat. Add the sliced sausage and sear for approximately 5 to 6 minutes, turning occasionally, until each side develops a golden-brown crust. Once browned, transfer the sausage to a plate and set aside.
In the same pan, pour in the remaining tablespoon of oil or butter. Toss in the diced potatoes and season them with salt, garlic powder, smoked paprika if using, and black pepper. Stir well to coat evenly. Let them cook uncovered for about 15 to 20 minutes, stirring intermittently. For a softer interior, cover the pan during the first 10 minutes to allow the steam to tenderize them, then uncover to let the exterior crisp.
Introduce the sliced onions to the skillet and continue cooking alongside the potatoes for 5 to 7 minutes more, until the onions become tender and lightly caramelized.
Return the reserved sausage to the skillet. Stir thoroughly to blend the components and heat everything through for another 2 to 3 minutes, allowing the flavors to meld.
Sprinkle with chopped fresh parsley, if desired, and serve the dish hot.