Clean and rinse the seafood. Remove any visible bones from the fish, trimming the "beards" off the mussels and discarding any shellfish that has opened.
Peel and slice the onion thinly into slivers. Divide in half.
Submerge one half of the sliced onions into a bowl of ice water. Reserve the remainder to use when poaching the batches of seafood.
Place two cups of water in a saucepan and heat to boiling. Add one third of the reserved sliced onions and 1/8 teaspoon salt to the water. Add the fish filet to the skillet, reduce heat and poach for 3-5 minutes until done.
Once the fish is cooked, remove it from the water with a slotted spoon and place on a separate dish to cool.
Drain the water (or reserve for stock) and replace it with fresh water. Add salt and one third of the sliced onions and poach the squid, octopus, scallops or lobster (or combination of seafood you're using) for 5 minutes.
Remove with a slotted spoon, drain and cool the seafood at room temperature.
Drain the water (or reserve for stock) and replace it with fresh water. Add salt and the final third of the reserved sliced onions, cover and poach the mussels (clams or preferred shellfish ) for 5 minutes or until opened. Discard any that don't open.
Remove the mussel meat from the shells. Cool at room temperature.
Peel the cucumber, seed and dice into small pieces.
Seed and dice the red and green peppers into small pieces.
Whisk the freshly-squeezed lime juice, naranja agria (or a mixture of orange juice and grapefruit juice) and vegetable oil with remaining salt in a large bowl.
Chop the cooked and room temperature fish and seafood into medium-sized pieces. They should be in bite-sized pieces but still recognizable. You can leave the squid rings whole if they're not too large.
Add the seafood to the bowl along with the reserved slivered onions (drained from ice water) with the vinaigrette dressing. Add chopped cilantro and salt and pepper. Toss lightly.
Cover and place in refrigerator to chill for at least an hour.
Remove from the refrigerator. Adjust seasoning to taste.
Serve chilled in individual tapas dishes, with seafood forks and crostini or slices of baguette on the side.