Cook bacon in a large wide pot over medium-high heat until just crisp, about 5 minutes. Add sugar and cook, stirring constantly, for 30 seconds more. Add onions and salt and pepper and cook, stirring occasionally, until onions are golden and soft, about 10 minutes. Stir in apples, reduce heat to medium-low, cover, and cook until tender, 6–8 minutes.
Add port, vinegar, and cabbage to the onion–apple mixture; stir to combine. Cover the pot and cook until cabbage is bright purple and slightly wilted, 5–7 minutes. Add stock and salt and pepper. Increase heat to medium-high and bring to a boil. Reduce heat to medium-low, cover, and cook until cabbage is tender but still red, about 1 1⁄4 hours. Stir in red currant jelly, season with more salt and pepper, and cook briefly, 4–5 minutes more.