Heat 1 tablespoon of olive oil in a large skillet or cast-iron pan over medium-high heat until shimmering. Add the sliced kielbasa in a single layer and cook undisturbed for 2–3 minutes until deeply browned on one side. Flip and brown the other side for another 1–2 minutes. Transfer the sausage to a plate and set aside.

Reduce heat to medium and add the remaining 1 tablespoon of olive oil to the same pan. Add the sliced onion and cook, stirring occasionally, for 4–5 minutes until softened and starting to turn golden. Scrape up any browned bits from the sausage as you stir — that's flavor.

Add the minced garlic, smoked paprika, onion powder, and red pepper flakes (if using) to the pan. Stir and cook for 30–60 seconds until the garlic is fragrant. Do not let it burn.

Add all the cabbage to the pan at once. It will look like a lot — that's fine, it wilts down significantly. Season with salt and black pepper. Toss to coat with the onion and spice mixture. Cook over medium to medium-high heat for 12–15 minutes, stirring every couple of minutes but allowing the cabbage to sit and caramelize between stirs. You want golden, slightly charred edges on some pieces — that's the goal.

Return the browned sausage to the pan and toss everything together. Cook for another 2–3 minutes to heat the sausage through and let the flavors meld.

Remove from heat and drizzle the apple cider vinegar over the skillet. Toss to combine. Taste and adjust salt and pepper as needed. Garnish with fresh chopped parsley and serve immediately directly from the pan.