Finely grate the zest from the lemon into a small bowl. Squeeze in the juice from half the lemon. Peel and grate in the garlic and ginger. Halve the chilli, scoop out the white seeds and pith and finely chop the chilli. Add the chilli to the bowl.
Add the mustard, caster sugar and sesame oil to the bowl with a pinch of salt and 1 tbsp olive oil. Whisk together to combine and make a dressing. Taste and add a squeeze more lemon juice, a splash more olive oil or a pinch of salt or freshly ground pepper if you think it needs it. Set aside.
Put a medium pan of water on to boil. Set a frying pan over a medium heat for 1 min, then add the sesame seeds. Toast for 2-3 mins, stirring often, till the sesame seeds are golden. Tip into a large bowl.
Trim the roots and top 3cm off the top of the spring onions. Finely slice the spring onions on the diagonal. Add them to the sesame seeds. Trim the woody ends off the sugar snap peas.
When the water in the pan is boiling, add the sugar snap peas (keep them whole, you can eat the pods). Simmer for 2 mins to just blanch them, then use a slotted spoon to scoop them out of the water (keep the water in the pan for later). Pop the sugar snaps in a sieve and rinse briefly under cold water to stop them cooking. Add to the bowl.
Separate the pak choi leaves and slice the larger leaves in half or quarters. Get the pan of water back to the boil, then add the pak choi leaves. Simmer for 2 mins to just blanch the leaves. Drain and rinse under cold water. Shake off any excess water and tip the pak choi leaves into the bowl.
Whisk the dressing again to make sure it is combined, then add to the bowl and turn everything to ensure it's coated in the dressing and the sesame seeds. Divide the salad between 2 warm plates and serve.