Preheat the grill or oven to 400°F (200°C).
In a large bowl, combine steak cubes, potatoes, and sliced onion.
Add olive oil, melted butter, garlic, Italian seasoning, paprika, salt, and black pepper. Toss everything well until evenly coated.
Divide the mixture evenly between large foil sheets.
Fold the foil tightly around the steak and potatoes to create sealed packs.
Place the foil packs on the grill or a baking sheet in the oven.
Cook for about 25–30 minutes, carefully flipping halfway through if grilling.
Open the foil carefully once the potatoes are tender and the steak is cooked to your liking.
Garnish with fresh parsley before serving.