Remove fish fillets from fridge or freezer to come to room temperature.
Melt butter in a pan on low heat and add lemongrass stalk.
Add coriander and mustard seeds and stir to ensure that the butter is not burning.
Once the seeds start to splutter, stir for another minute and then take out the lemongrass stalk and the seeds from the pan.
Add gari to the melted now lemongrass-infused butter and stir for about a minute.
Add all the herbs, spices (except black and white pepper) and then stir together for another minute.
Taste for salt, let cool completely and heat oven to about 180°C-200°C.
Pat fish dry and season with salt, black pepper and white pepper and place fillets on a tray lined with baking paper.
Take about 2 tablespoons of the gari mixture and press firmly on top of each of the fillets.
Bake fillets for approx 20-25 minutes or until the fish begins to flake when you put a fork in it.
Serve warm.