Combine prawns, chilli, lemon zest/juice, garlic, 2 tablespoons olive oil, and coriander in a bowl.
Marinate the prawn mixture in the refrigerator for at least two hours.
Griddle the prawns on a hot griddle pan for 1-2 minutes per side until pink.
Blanch asparagus in boiling water for 1-2 minutes until tender, then drain.
Toss asparagus in remaining oil and griddle for 1-2 minutes.
Griddle lemon halves cut-side down for one minute.
Process egg yolks, vinegar, mustard, and juice from one charred lemon in a food processor.
Gradually drizzle in rapeseed oil with the motor running until the mayonnaise thickens.
Season mayonnaise with salt and black pepper.
Arrange asparagus and prawns on plates with watercress, lemon mayonnaise, and a lemon half.