Put the cabbage in a large saucepan or dutch oven and cover it with water.
Add a splash of vinegar.
Bring to a heavy simmer and cook until tender, about 20 minutes.
In the meantime, caramelize the onions and garlic in a couple tablespoons of olive oil seasoned with a pinch of red pepper flakes in your largest skillet.
Add the apples to the skillet when the onions have changed color and have softened.
When the onions are caramelized and the cabbage and apples are tender, use a slotted spoon to transfer the cabbage to the skillet with the onions.
Stir well to combine and cook off any excess liquid.
Salt and pepper to taste.