Fill a large bowl with ice and water. Bring a pot of water to a boil, and add 1 teaspoon salt and string beans. Cook until bright green, about 3 minutes, and drain into a colander. Transfer to ice bath to stop cooking, drain again, and set aside.
Place a large skillet over high heat, and add oil. When very hot, add mustard seeds, and cook until seeds begin to pop, about 30 seconds. Add onion, and cook, stirring until they begin to brown, 3 to 4 minutes. Add ginger, and cook 1 minute more. Add reserved string beans, and cook, stirring until hot. Season with salt to taste.