Wash the spinach, then put it in a saucepan with just the water clinging to the leaves from washing. Add the chilli flakes and cook gently for 3–5 minutes until just wilted. Drain thoroughly, then transfer to a serving bowl.
Sprinkle the spinach with the sesame oil, salt and lemon juice and toss to mix. Sprinkle with toasted sesame seeds, then serve at once.