Prepare all the seafood by first opening the oysters into a sieve over a bowl to catch the juices (see video technique). Trim and slice the salmon into 6 equal-size square chunks. If the scallops are large, cut them in half lengthways. Peel and devein the prawns if needed.
Place the strained oyster juice and fish stock into a shallow saucepan and bring to a simmer. First add the salmon and poach for 1 min. Add the prawns and oysters (if using large ones) and poach for 1 min more. Add the scallops and poach for 1 min more, then add the oysters (if small) and simmer everything for a final min until just cooked.
Carefully tip the fish and poaching liquid into a sieve over a large saucepan or bowl, keeping all the salmon chunks intact.
Heat half the butter in the cleaned shallow pan and add the shallot. Cook very gently for 5 mins until soft but not coloured, then pour in the alcohol and boil until reduced to a few tablespoons. Pour in the poaching liquid and boil down until reduced by half, about 20-25 mins. Stir in the cream and, once more, reduce by half. Turn down to a gentle simmer and whisk in the rest of the butter.
Gently add the seafood to the sauce, taking care not to break up the salmon. Simmer lightly until heated through, then add most of the herbs, squeeze over the lemon juice and set aside.
Meanwhile, cook the spaghetti in plenty of boiling water until just done, about 3-4 mins, then drain well and add back to the pan with a drizzle of olive oil and the rest of the herbs. You are now ready to plate up.
Use a roasting fork to twirl a neat bundle of spaghetti. Stand the bundle in the middle of a warm large pasta bowl. Alternate the salmon and seafood around the pasta. Spoon the sauce over the seafood, top the pasta with the chervil or parsley sprigs and serve immediately.