Wash the mustard greens and spin dry. Discard any dodgy leaves and cut out thick stalks.
Heat a wok or large frying pan until very hot. Add the olive oil and swirl to coat the base. As soon as the oil starts to smoke, throw in the greens and toss, turning with tongs until uniformly wilted, which will take about 2 minutes, depending on the size of your pan and its temperature.