Meanwhile, in a large (12-inch) nonstick stir-fry pan, heat the oil over medium heat. Add the diced turnips and radishes and 1/4 teaspoon of salt. Cook, stirring, until all the veggies are slightly browned, about 5 to 6 minutes. Add 1/4 cup of the scallions and the ginger, garlic, and red pepper flakes. Cook, stirring, until fragrant and softened, about 1 minute. Add the 2 cups of chopped greens and cook, stirring, until wilted, about 30 seconds. Remove the pan from the heat.