Cast iron not recommended due to the acidity of the vinegar, which may react to the cast iron.
1 large yellow onion, thinly sliced
1 tablespoon coconut oil
1/2 head of red cabbage, thinly sliced
2-4 tablespoons unfiltered apple cider vinegar
1 teaspoon fresh rosemary, minced
½ teaspoon salt
1 Fuji apple, sliced into matchstick-sized pieces