In a food processor or blender, combine the onion, garlic, ginger, and green chilies. Bend the mixture until it is pureed.
Heat the oil in a deep pan, add the curry leaves, garden cress seed, onion, ginger, garlic, green chilli, and cashew paste, and sauté for a few minutes.
Saute the remaining spices and chicken for 3–5 minutes, or until most of the water has evaporated. Cook till almost done, then add the large pieces of Kerala cucumber and a cup of heated water.
Add salt and coconut milk and cook till the chicken and kerala cucumber are tender.
Remove from the heat and serve immediately with red rice.