Trim and clean the tatsoi then spin it dry.
Cut the bulb portion of the scallions off and mince those. Thinly slice the green portions.
Combine the scallions, lemon juice, mustard and salt then stir it well until the salt dissolves. Blend in the oil.
Pour the dressing into a wide skill over medium heat. Add the tatsoi leaves and turn them gently to coat the leaves. Cook the mixture until the leaves wilt and the stems are still crunchy. This should take about 2 minutes.
Place in a nice serving dish for presentation.