Cook the onion in a large, heavy-based frying pan over medium heat until translucent. Make sure to add a bit of water if the onion begins to stick too much and take on colour.
Add the chili powder and tomatoes and cook for 10 minutes.
Ladle 2-3 tablespoons of the tomato sauce into a small bowl and gradually mix in the almond butter to create a smooth sauce, making sure it’s not too thick and dry – if it is, just add some more tomato sauce
Pour the contents of the pan into the tomato sauce and mix well.
Add the fish to the sauce and cook for 10-15 minutes, the fish will break up, but this is fine.