Boil the asparagus spears in salted water for about 4–5 minutes, or until they are cooked but still slightly firm. Drain them but keep the water. If the ends of the asparagus are still hard and woody, cut these away. Cut off the asparagus tips (if they are large, cut them in half lengthways) and keep them on one side. Roughly chop the remaining asparagus.
Heat the oil and half the butte