1. The evening before, combine the pesto, olive oil, vinegar and mustard, if using, in a large mixing bowl
2. Add the cooked potatoes along with the tomatoes and cucumber. Check the seasoning and adjust if necessary.
3. Divide the salad between 2 airtight containers and either place the beef on top or pack it separately, according to taste. Refrigerate overnight; the second portion will keep for up to 2 days.