Warm a dry deep frying pan or wok over a low heat. Measure out 1 tsp of the coriander seeds. Add them to the pan. Toast for 1-2 mins till they smell nutty. Tip them into a bowl.
Roughly crush the coriander seeds with the end of a rolling pin or the bottom of a jar. Pop them into a small food processor (see our tip if you don't have one). Roughly chop 1 of the chillies, scraping out the seeds and white bits for less heat. Add the chilli to the processor.
Peel and roughly chop the garlic and ginger. Add them to the processor. Pick the leaves off the coriander and pop to one side to use as a garnish later. Roughly chop the stalks. Add them to the processor with 2 tbsp cold water. Blitz the chilli mix in the processor to make a rough paste.
Trim the courgettes and use a spiraliser to turn them into courgette spaghetti. If you don't have a spiraliser, use a veg peeler to peel thick ribbons off them. Snap the woody ends off the asparagus. Chop into chunks around 3 cm long.
Warm 2 tsp olive oil in the deep frying pan or wok. Add the chilli paste. Stir and fry over a medium heat for 2 mins till the pan smells spicy. Pour in 500ml boiling water. Add the coconut milk. Bring to the boil. Lower the heat.
Add the asparagus to the pan. Simmer for 2-3 mins till just tender. Stir in the courgette ribbons and the crab. Simmer for 3-5 mins till the veg are just tender. Taste and add salt and pepper if it needs it.
Finely slice the remaining chilli. Ladle the laksa into warm bowls. Top with chopped coriander leaves and sliced chilli to serve.