Squeeze lemons and add 1 tablespoon of lemon juice to 1 litre of boiling salted water and bring to boil.
Cut squid into ½ inch rings. Add to the water and simmer for 6 –8 minutes. Leave to cool in the water.
Wash the mussels. Melt some butter in a pot and add some chopped onion. Cook for 2 minutes and add the mussels and half a cup of white wine. Allow the mussels to steam for a few minutes. Remove from the pot and allow to cool. Drain the squid and add to the mussels and the prawns in bowl.
Deseed the tomatoes and cut flesh into long thin strips. Peel the onion, finely chop and put into a bowl with the tomatoes. Peel the garlic and crush and add to the mixture. Rinse and dry the herbs. Finely chop parsley and basil and add to the bowl.
Mix in the rest of the lemon juice with salt, pepper, balsamic vinegar and olive oil and pour over the salad, mix well and put in the fridge for about 1 hour.
Serve with watercress and garnish with sprig of parsley.