Rinse chicken breast with cold water and cut into bite-sized pieces. Marinate in soy sauce. In a saucepan, combine coconut milk with 750 ml (approximately 3 cups) water. Peel and finely chop ginger. Slice chile pepper into rings. Add ginger, chile, lime leaves and lemongrass to the saucepan and bring to a boil. Rinse, trim and cut mushrooms into slices.
Add mushrooms to soup. Add chicken and simmer about 7 minutes. Season soup with a little lemon juice, salt and sugar to taste. Rinse, quarter and remove seeds from tomato, then slice thinly and stir into soup. Divide soup among bowls, garnish with cilantro sprigs and serve immediately.