Rinse fish and squid and pat dry. Parboil squid in salted water, about 20 minutes. Sprinkle fish with lemon juice. Heat 4 tablespoons olive oil in a pan. Add fish and fry on both sides. Season with salt and pepper. Cook squid in the same manner and set aside.
Drain beans. Rinse, halve and remove seeds from tomatoes, then cut into cubes. Rinse and slice celery. Peel onion and cut into thin strips.
Mix beans, olives, tomato, onion and celery with 5 tablespoons olive oil and 3 tablespoons white balsamic vinegar. Season with salt and pepper. Stir in basil leaves and let marinate about 1 hour. Stir in fish and squid and let marinate about 30 minutes. Serve cold as a small appetizer, with fresh white bread if desired.