Put a large, deep saucepan over a medium-high heat. Once hot, add one of the coconut cream tins and cook until the oil separates and the coconut solids turn golden.
Meanwhile, pick the leaves off the coriander, set aside.
Add the curry paste and coriander stems to the coconut oil, then cook, stirring, for 2 mins. Pour the chicken stock in along with enough water to fill the saucepan, then bring to the boil. Turn the heat down to medium and simmer for 10-15 mins to make a broth (skim off any foam that comes up).
Meanwhile, cook the noodles following pack instructions. Trim and thinly slice the radishes (use a mandoline if you have one).
Strain the broth through a fine mesh sieve, then stir through the remaining coconut cream and watercress until wilted. Season with fish sauce, lime juice and black pepper.
Divide the noodles and soup between bowls, then top with a drizzle of chilli oil, radishes, crispy shallots and coriander leaves to serve.
