Finely chop the garlic, chilli and lemongrass and put into a bowl with the other ingredients, except for the olive oil and only one of the teaspoons of cumin. Mix well and taste adding more sugar or salt as needed.
Rub the salmon with olive oil and the second teaspoon of cumin. Spoon the marinade over the fillet pressing the solid bits of garlic, lemongrass, chilli and lime zest to make a crust on the salmon.
Leave to rest for about 15 mins or as long as you have. It isn't necessary but I find it improves the flavour.
Cook on gas 6 or at 200 C for about 15 minutes. If it looks done and flakes with a fork, take it out sooner.