Heat a large, heavy skillet over medium heat and add the vegetable oil and unsalted butter. Let the butter melt completely and swirl the pan so the bottom is evenly coated.

Add the cubed russet potatoes to the skillet in an even layer. Cook without stirring for 4–5 minutes, until the bottoms start to turn lightly golden.

Sprinkle the potatoes with half of the fine sea salt and half of the black pepper. Gently stir and continue cooking for another 5–6 minutes, stirring only every couple of minutes so the potatoes can brown.

Pour in the water, quickly cover the skillet with a lid, and let the potatoes steam for 4–5 minutes over medium-low heat, until they are just tender when pierced with a fork.

Remove the lid and add the sliced smoked Polish sausage to the skillet. Cook for 4–5 minutes, stirring occasionally, until the sausage starts to brown around the edges.

Add the thinly sliced yellow onion to the skillet and sprinkle in the garlic. Cook for 5–6 minutes, stirring often, until the onion softens and begins to caramelize lightly.

Sprinkle the smoked paprika, dried thyme, remaining fine sea salt, and remaining black pepper over the potato mixture. Stir well so the spices coat the potatoes, onions, and sausage evenly.

Continue cooking for another 3–4 minutes, stirring occasionally, until the potatoes are deep golden and slightly crisp on the edges and everything is heated through.

Remove the skillet from the heat. Taste and adjust seasoning if needed. Sprinkle the chopped fresh parsley over the top, if using, and serve the fried potatoes with onions and smoked Polish sausage hot.