In a large skillet over medium-high heat, add the olive oil. Once hot, add the sliced kielbasa. Cook until the sausage is nicely browned on both sides, about 5-7 minutes. Remove the kielbasa from the skillet and set aside.

In the same skillet, add the butter (if using) and onions. Sauté until the onions start to soften and turn translucent, about 3-4 minutes. Add the minced garlic and cook for another minute until fragrant.

Add the sliced cabbage to the skillet. Season with salt, pepper, caraway seeds (if using), and red pepper flakes. Stir well to combine with the onions and garlic.

Pour in the chicken broth or water and apple cider vinegar. Stir to mix, scraping up any browned bits from the bottom of the skillet.

Return the browned kielbasa to the skillet, nestling it into the cabbage. Reduce the heat to medium-low, cover, and let simmer for about 10-15 minutes, or until the cabbage is tender and the flavors have melded together.

Once the cabbage is cooked to your liking, taste and adjust the seasonings as needed. Serve hot directly from the skillet.