Cook the prepared Frutti di Mare in a little salted water. While still warm, dress with olive oil, the juice of 1/2 lemon, salt and pepper.
Cut half of the avocado lengthwise into thin slices, sprinkle with the juice of one lemon to prevent discoloration, and spread evenly on four plates as a garnish. Pile the leaf lettuce next to each. Mash the rest of the avocado in a hand blender, adding the chopped shallot, season with enough pepper, juice of one lemon, salt and parsley.
Place the mashed avocado as heaps in between. Decorate with tomato strips.