1) Preheat the griddle to medium-high.
2) In a medium bowl, add the orange juice and the lime juice. Stir in the achiote paste, cumin, oregano and salt. Mix into a smooth paste. Rub the paste all over the inside and outside of the fish. Drizzle both sides of the fish liberally with vegetable or rapeseed oil.
3) Put the fish on the griddle. Griddle until golden, about 4 minutes, and then carefully flip the fish over. Griddle until the fish starts to separate when lifted with a spatula. The fish will be golden brown with some charring.
Lay the fish on a large platter. Cover it with pickled red onions, sliced avocado and chopped coriander and serve.
Pickled red onions:
Add the sliced onions to a medium bowl large enough for the onions to be spread out into a thin layer. Juice the limes into the bowl, then add the chopped chilli and salt. Stir to combine. Allow to sit at room temperature for 2 hours, stirring every 30 minutes or cover and refrigerate overnight. The onions are ready when they are bright pink and softened. Their flavor will be sweet and spicy with a little texture but not crunchy.
Yield: About 1 cup