Fill a kettle and boil it. Finely grate the zest from the orange. Juice it. Mix the orange juice and zest, half the tamari and the Demerara sugar together. Taste. Add more tamari if you think it needs it.
Fill a pan with the hot water from the kettle. Add the noodles. Simmer for 3 mins. Drain the noodles. Rinse them under cold running water. Tip into a bowl. Cover with cold water. Pop to one side.
Drain the tofu. Pat dry. Slice it into cubes about 1½ cm big. Slice the celery into thin crescents. Trim and finely slice the spring onions. Trim and peel the carrots. Use a peeler to slice very thin strips off them, turning as you go.
Warm 1 tbsp olive oil in a deep frying pan or wok. Add the tofu. Fry for 5 mins. Turn the tofu every so often so each side browns. Lift out of the pan. Pop on a plate lined with kitchen paper.
Pick out any large stalks from the watercress. Pop the celery, onion and carrot strips in a bowl together. Drain the noodles and add them, along with the watercress.
Wipe the pan dry with kitchen paper. Add the sesame seeds. Toast them over a lowish heat for 2 mins till they’re golden. Keep shaking the pan till they’re cooked.
Add half the sesame seeds to the noodles with the dress. Use to forks to pick up and toss everything together. Pop on two plates. Top with the tofu and the rest of the sesame seeds.