Peel and mince the 2 garlic cloves. Halve the red chilli lengthways, remove the seeds, and finely chop it. Using a mortar and pestle, grind the garlic, chopped chilli, and a bit of salt together into a paste.
Rinse the cod fillet, pat dry, and cut into 3 cm cubes. Clean the shrimp. In a bowl, combine the fish, shrimp, 3 tablespoons of lemon or lime juice, and the garlic-chilli paste. Cover and refrigerate.
Peel and finely chop the large onion. Rinse the tomato, remove the stem, and chop into small pieces. Rinse the coriander, pluck the leaves, and finely chop them.
Heat 1 tablespoon oil in a large saucepan over medium-high heat. Add the chopped onion and cook for 1 minute, stirring. Add the chopped tomato and 100 ml of the coconut milk. Stir frequently and cook for 2 minutes over medium-high heat.
Add the fish, shrimp, and marinating liquid to the saucepan. Pour in the remaining 100 ml of coconut milk. Stir, reduce heat to medium, cover, and cook for 5 minutes until the fish flakes easily with a fork.
Stir in the chopped coriander and the remaining 1 tablespoon of lemon or lime juice. Season with salt to taste.