Heat a large cast-iron skillet or pan over medium-high. Add the kielbasa coins and cook until golden brown and slightly charred, about 3 minutes on each side.
Add butter, onion, garlic, 1 teaspoon salt, ½ teaspoon black pepper and a pinch of crushed red pepper and cook, stirring, until the onion is starting to brown and the garlic is fragrant, about 3 minutes.
Add cabbage and cook, stirring occasionally, until most of the cabbage has softened and browned, 10 to 12 minutes.
Add the vinegar and mustard and stir to incorporate, cooking for 1 minute longer. Taste and adjust for seasonings.
Garnish with chopped parsley and serve immediately, with more mustard on the side.
