Heat a saute pan over high heat. When the pan is hot, add 1 tablespoon of the oil. Add 2 teaspoons of the scallions, the ginger, 1 tablespoon garlic, and red pepper flakes and cook for 1 minute, stirring constantly. Add the carrots, radishes, and turnips and toss to combine. Add the water and saute for 3 minutes over high heat. Add the remaining vegetables and saute for 3 more minutes. The vegetables should be slightly undercooked and still have some crunch. Season with black pepper. Push the vegetables to 1 side of the pan. In the open portion of the pan add another 1 teaspoon of oil. Add another 2 teaspoons of garlic, an additional pinch chile flakes, and remaining 1 teaspoon of the scallions and stir just to soften. Toss everything together with the vegetables. Add the soy sauce and toss to combine. Add 1/4 cup spinach leaves and toss for 1 minute. Dissolve the cornstarch in the water to make a slurry. Add a little of the slurry and bring just to a boil to thicken. Remove from the heat and serve.