Your recipe is similar to Delia's MGM, except Delia uses olive oil not butter, and adds a clove of garlic, crushed, a few mins before the onions are just browned. Oh, and adds a few black peppercorns. She leaves it on a low heat for 15 mins, but if your pot is a good heat-retaining one it'll come to much the same.
You've reminded me though of all the bubble and squeak and other fried cabbage delights
One of my faves, especially to go wiith sausages, is to fry onions in animal fat - the sausage juices work very well, or bacon grease - till softened, add shredded cabbage and cook till almost burned. If you used lard or olive oil, add some sausage juices and stir through, just before serving. Serve with tomato ketchup. I add peas, too, if I have some.
Ideal with fried potato patties. Heart attack on a plate, but you'll die with a smile on your face.