Place yakisoba noodles on a microwave-safe plate, cover loosely with plastic wrap, and microwave at 600W for about 1 minute to loosen. Remove pits from umeboshi and mash them. Drain the tuna can. Cut chives into 4 cm pieces and slice green onion diagonally.

Heat half of the salad oil in a frying pan, add bean sprouts, chives, and green onion, and stir-fry briefly. Remove from the pan.

Add the remaining salad oil to the pan, then add yakisoba noodles and stir-fry over medium heat. Add water and steam-fry until the water evaporates.

Return the vegetables to the pan, add tuna and chicken stock powder, and stir-fry together.