For the court-bouillon, add all ingredients into a large stockpot and bring to the boil. Reduce to a simmer and cook on for 30 minutes. Remove from the heat and pass through a sieve. Place a clean poaching dish or roasting tin on the hob and transfer the stock in. Bring up to the boil, then reduce back down to a simmer before adding the salmon fillets, working in batches if you need to. Place a lemon slice on top of each and cook on a low heat for 4 minutes (a bit longer if a larger fillet), or until the fish is no longer translucent. Turn off the heat, cover with tin foil and allow to cool. Refrigerate before serving. For the dressing, add all the ingredients to a mason jar and shake well to emulsify. When ready to serve remove the salmon from the liquid and spoon the dressing over the salmon fillets