Heat the oil in a large saucepan over medium-high heat. Add the ginger, garlic, and scallion whites and cook, stirring frequently, until softened, 2 to 3 minutes. Stir in the curry powder. Add the carrots, potato, lentils, broth, ¾ teaspoon salt, and ¼ teaspoon pepper. Bring to a boil, reduce heat, and simmer, stirring occasionally, until the lentils and vegetables are tender, 15 to 20 minutes.