Lightly season the salmon. Heat the oven to 200C/fan 180C/gas 6. Put the oil, onion, garlic, ginger and some seasoning in a pan over a medium heat, and fry for 5 minutes or until translucent. Add the chilli and lemongrass, and fry for another minute. Stir in the curry paste and mix for 1 minute. Splash in the soy sauce followed by the chopped tomatoes, coconut milk and lime zest, and mix together.