In a bowl combine the tandoori paste and ¼ cup of the yoghurt. Smear over the salmon and marinate in the fridge for a minimum 2 hours, but ideally overnight.
Transfer the salmon to a baking paper lined baking tray and bake in 200°C oven for 10 minutes until just cooked. Allow to cool and transfer to serving dish.
Combine the white vinegar, sugar and salt in a bowl and microwave for 30 seconds or until the sugar dissolves.
Pour over the sliced onion and set aside for 20 minutes, stirring occasionally until the onion softens.
Put the remaining yoghurt in a bowl with the lemon zest and juice.
Finely chop ¼ cup coriander leaves and add to the yoghurt with salt and pepper to taste.
Place the watercress, cucumber, radishes and remaining coriander around the salmon, scatter the onion over the salmon, put the yoghurt dressing alongside and serve.