Put your oven on preheat of 230 C. Meanwhile rub the olive oil on the beef and then let it roast in the oven for half an hour. When done, take it out and let it cool. Cut it into narrow strips.
Make the dressing now. Whisk together the vinegar, olive oil, mustard, caster sugar, salt and pepper in a large mixing bowl. Add the beef strips to this dressing and let it cool in the refrigerator.
To add to the salad, boil potatoes in a pan until tender. Let them cool and then cut into small pieces to add to the beef. Boil the beans and peas and add them to the beef too.
Remove the salad from the refrigerator a little while before serving and stir well.