These marinated scallions are a take on Japanese tsukemono (Japanese preserved vegetables) and make a wonderful side dish or sandwich topping!
Whisk the arrowroot starch with a small amount of the water in a small saucepan. Once that is mixed, slowly whisk in the rest of the water. Bring the water to a boil over medium-high heat, then reduce the heat to maintain a simmer and continue simmering for 5 minutes. After 5 minutes, remove the saucepan from the heat and pour the liquid into a heat resistant container, like a glass jar and let it cool until it's warm. Cover the container with a lid and refrigerate. Once it is chilled, it is ready to use.
Once the arrowroot mixture has chilled, whisk in the rice vinegar, whole grain mustard, salt and pepper, then set aside.
In a saucepan fitted with a steamer basket, bring 1 inch of water to a boil. Add the scallions to the steamer basket, cover and steam until just tender. Steaming time will vary depending on how thick your scallions are, but begin checking them after 3 minutes of steaming time.
Drain the scallions and put them into a container for marinating. Pour the marinade immediately over the scallions and gently stir so that the scallions are completely coated. Let the scallions cool before putting a lid on the container and putting it in the refrigerator to chill. The scallions are ready to eat once chilled. They will keep in the refrigerator for about a week.