Brush and wash mussels thoroughly. Carefully wash clams several times under running water to remove all sand residue. Separate mussels from clams and force both to open in covered pots over high heat with no seasoning. Remove shells. Strain cooking juices through a fine sieve and pour over the mussels and clams. Set aside.
In a pot with one quart of boiling water, cook herbs and vegetables (onion, celery, carrot, rosemary, marjoram, thyme) for about 20 minutes and strain. Return strained broth to the pot, bring to a boil. Cook shrimp in broth for about 3 minutes, then peel.
Use the same broth to cook baby octopi (cleaned, washed and cut into pieces) and baby squid for about 15-20 minutes, until tender. Arrange seafood on a large serving platter. On the side, combine oil, the juice of one lemon, garlic (leave whole for fragrance only or chop for more intensity), lots of chopped parsley, a pinch of salt and a dash of pepper.
Mix with a fork and pour over the seafood. Toss the salad, cover and refrigerate for at least 30 minutes before serving. Make a sauce with oil, a teaspoon of the juice of one lemon (or more if necessary), the whole or chopped cloves of garlic, a generous amount of chopped parsley, salt and pepper.
Make an emulsion by whisking with a fork and pour into the bowl. Mix, put a cover on and keep in a cool place for at least 30 minutes.