Crush the lemongrass with the back of a knife to release the flavour. Peel the ginger with the back of a spoon and slice into thin wide pieces. Add to a pan along with all the other stock ingredients and bring to the boil. Turn down to a gentle simmer. Dice the salmon into cubes and thread onto a cocktail stick. Poach in the broth for 2 to 3 minutes depending on how you like your salmon cooked.
Peel and dice the onion, garlic, celery and carrot. Sweat in olive oil for around 5 minutes adding a pinch of maldon sea salt if starts to catch. Add the lentils, bay leaf and thyme, cover with stock and simmer until the lentils are tender. Add more stock during the cooking if required. Finish with a knob of butter, a squeeze of lemon, the seasoning and freshly chopped parsley.
In a bowl whisk together the vinegar, mustard, salt and pepper, Slowly whisk in the oil so the vinaigrette starts to emulsify (get thicker). When you have the correct consistency finish with the tarragon. Pour in to a squeezy bottle.