In a large pestle and mortar pulverise the peeled garlic cloves with the salt, so that it makes a smooth paste. Then add the chopped and seeded chilli and crush again until you have a gloriously red-tinged mixture.
Tip in the crab meat, breaking it up gently with a fork, and pour in the oil. Zest the lemon into the mortar and then add the juice. Using a fork, beat well to mix, and then you are ready to cook your pasta
Drain the pasta and tip into a warmed serving bowl. Immediately pour over the crab sauce and toss the pasta about in it, then throw in the parsley and watercress and toss again