For the prawns, mix the prawns, chilli, lemon zest and juice, garlic, two tablespoons of the olive oil and coriander in a bowl until combined. Marinade in the fridge for at least two hours.
Heat a griddle pan until hot and griddle the prawns for 1-2 minutes on each side, or until pink and just cooked through.
Blanch the asparagus for 1-2 minutes in a pan of boiling water, or until just tender. Drain and toss in the remaining olive oil then griddle on the griddle pan for 1-2 minutes.
For the roasted lemon mayonnaise, heat a griddle pan until hot and griddle the lemon halves, cut-side down, for one minute.
Blend the egg yolks, vinegar and mustard into a food processor, then squeeze in the juice from one of the charred lemons and blend until well combined. While the motor is running, gradually add the oil. Continue until all the oil has been added and the mayonnaise has thickened. Season with salt and freshly ground black pepper
To serve, arrange the asparagus on serving plates and top with the prawns. Place the watercress alongside and finish with a spoonful of lemon mayonnaise and half a griddled lemon.