Put the coconut milk, ginger, lemon grass and shallots in a large saucepan and bring to the boil. Reduce the heat to a simmer, add the chicken, sugar and fish sauce and cook for 5 minutes or so, or until the chicken is cooked through.
Add the mushrooms and tomatoes and cook for a further 3–4 minutes.
Add the lime juice, lime leaves or zest and the chilli