Peel and slice the onion. Click off and discard any tatty outer leaves from the cabbage, then remove the core and chop into irregular chunks.
Peel the apples, then chop into 2.5cm pieces, and pick and finely chop the parsley leaves. Finely slice the bacon.
Drizzle a good lug of oil into a pan over a medium heat, add the bacon, then bash and add the fennel seeds and cook until golden.
Add the onion, cover, and continue to cook for 5 to 10 minutes, or until lightly golden and sticky.
Scatter in the apple and cabbage, season with sea salt, black pepper and the vinegar, then stir well.
Pop the lid back on, reduce the heat to low and cook gently for 1 hour, or until gorgeously sticky and sweet, stirring occasionally.
Scoop into a serving dish, pop the butter on top and sprinkle over the parsley, then serve.