Combine water, bay leave and thyme leaves in a small pot and bring to a boil. Peel and devein the prawns, reserving the prawn heads and shells. Add the prawn heads and shells into the boiling water. Simmer for 15 to 20 minute to extract the flavours of the prawns. Strain the stock into another clean pot.
Gently rub 1 teaspoon salt into the peeled and deveined prawns and set aside for 10 minutes. Rinse off the salt, pat dry with paper towels and set aside. Skin the salmon fillet (or you can leave it, which was what I did) and slice it into 1 1/2-inch chunks. Season with a little salt and set aside.
Reheat the stock until it starts to simmer. Add in the salmon chunks and simmer for 1 minute. Add in the prawns and the clams and simmer until the clams are cooked. Remove all the seafood with a slotted spoon and set aside.
Boil a large pot of well-salted water and cook the spaghetti until al-dente, according to package's instructions.
In a third pot, melt the butter over medium-low heat then add in the onion. Sauté gently until the onion is translucent, then add in the stock. Bring to a simmer and add in the cream and cherry tomatoes. Add in the seafood and season with salt and black pepper (I used very little salt, so remember to do a taste-test before seasoning the sauce!) Bring the sauce to a simmer but take note not to boil it.
By now your pasta should be cooked - drain well (no need to rinse under cold water). Take a bunch of pasta, twirl and plate it. Spoon the seafood and the sauce over the pasta. Garnish with parsley and serve immediately.