Step 1
To a large, wide pot, add the bacon, set over medium high-heat, and cook, stirring frequently, until the fat has rendered and the bacon is just beginning to crisp, about 5 minutes. Add the sugar and continue cooking, stirring continuously, until the sugar has liquified, about 30 seconds. Stir in the onions, season lightly with salt and black pepper, and continue cooking, stirring occasionally, until the onions are golden and soft, about 10 minutes. Stir in the apple, turn the heat down to medium-low, cover, and cook until the apple is tender, 6–8 minutes.
Step 2
Uncover the pot, stir in the Port, vinegar, and cabbage, then cover and cook until the cabbage is bright purple and slightly wilted, 5–7 minutes. Stir in the stock, season lightly with salt and black pepper, then turn the heat up to medium-high. Bring to a boil, turn the heat down to simmer, cover, and cook until the cabbage is tender but still bright purple, about 1¼ hours.
Step 3
Uncover the pot, stir in red currant jelly, and season to taste with more salt and black pepper. Continue cooking until the jelly is fully melted and the cooking juices are glossy, 4–5 minutes more. Remove from heat and serve hot.