Stem and chop the kale and mustard greens, discarding half of the stems and chopping the rest. Combine the miso paste, Sriracha and 2 tablespoons water in a small bowl. Set aside.
Heat the oil in a large saute pan over medium heat. Add the stems, cooking until tender, about 3 minutes. Add the chopped greens, pour the miso mixture over them and cook until the greens are tender, about 5 minutes.