Add butter to pan. When foam has subsided, add the onions and cook for about 5 minutes, stirring often.
Add cabbage, brown sugar, vinegar, salt and pepper and stir to combine. Reduce heat to low, cover the pan and braise for about 40 minutes, stirring occasionally. Adjust seasoning if needed.
To serve, sprinkle with reserved bacon and parsley, if desired.
Recipe by white plate blank slate at https://whiteplateblankslate.com/braised-red-cabbage-with-bacon/