Rub the sirloin steaks with salt and black pepper on both sides. Fry your steaks for a little less than however long you need to, to get them to the level that you enjoy them (blue / rare / medium-rare / medium / medium-well / well done / very well done).
In another pan, fry the diced red onions with the butter, until the onions have softened. Add the red wine and mustard and fry together for a further three to five minutes.
Add the steaks to the pan and cook together for a further few minutes (until your steak is cooked to how you like it). Serve up with delicious McCain Gourmet Sea Salt Chips!