Heat olive oil in a saucepan over medium low heat and sauté onion until soft. Add garlic and sauté for 2 minutes, then add green capsicum, tomatoes and coconut milk. Continue stirring with a spoon until milk begins to boil.
When milk begins to boil, add annatto powder, seasoning and prawns. Cook until prawns are pink and cooked through, about 5 minutes (do not overcook).
Remove from heat, sprinkle with chopped coriander and serve.
Also called achiote powder, it can be found at Asian or Latin American shops, or online. Annatto is mostly used for the vibrant colour it imparts, so in a pinch you could either omit it from the recipe or use equal parts paprika and turmeric instead.