Peel and halve the onions and cut into thin slices. Melt the butter in a large pan and sweat the onions in it over low heat for about 5 minutes. Now mix in the garlic and sugar and fry for another 2 minutes. Put the mixture on a plate lined with kitchen paper, season with salt and pepper and cover with aluminum foil to keep it warm.
Now heat the oil in the pan and fry the steaks in it for 4 minutes on both sides. Now brush the steaks with barbecue sauce and pour the onions over them. Fry the meat for another 3 minutes without turning it. The steak is now slightly bloody - if you prefer it medium, leave it in the pan for another 3 minutes.
Now sprinkle the chopped parsley over it and serve the steak.