Heat the oil in a large saucepan over a medium heat and cook the onion for 5 minutes, stirring occasionally.
Add the curry paste and garlic and cook, stirring, for another minute. Add the coconut milk, stock, fish sauce, sugar and salt, then bring to the boil. Add the red capsicum and zucchini, cover with a lid, then reduce the heat to medium and simmer for 10 minutes.
While the curry simmers, start to cook the rice by your preferred method.
Add the chicken to the curry and simmer, covered, for a further 10 minutes, then stir in the lemon juice and basil. Serve with rice.