In a medium pot, mix 1 1/2 cup water, dashi powder, soy sauce, and mirin, and bring to a boil. Add the frozen udon. Loosen the noodles with chopsticks, bring the soup back to a boil. Mix in the slurry and simmer until the soup is thickened. Swirl in a beaten egg slowly. When the egg is set, serve in a bowl and top with chopped green onion.