Heat the oil in a large, deep pan over medium heat and cook the chicken for 5-7 minutes until slightly golden. Remove it from the pan with a slotted spoon and set it aside.
Add the onion and red bell pepper to the same pan and cook for 4-5 minutes until the onion is translucent.
Next, stir in the garlic, ginger and lemongrass and cook for a further minute.
Add the Thai red curry paste and stir to combine. Cook for another minute, then add the zucchini and stir well so they’re covered in paste.
Stir in the coconut milk and simmer on medium-low heat for 5 minutes or until the sauce thickens.
Next, stir in the fish sauce and brown sugar, then return the cooked chicken back to the pan and toss well until well covered in sauce.
Simmer for 2-3 minutes, then add the lime juice and fresh cilantro.
Season to taste and serve hot over your favorite rice or with naan bread.