Heat 2 tablespoons of the oil in a large skillet over high heat.

Add scallops, sprinkle with 1/4 teaspoon of the salt and cook, stirring, until scallops have released their liquid and are cooked through, about 3 minutes. (If using sea scallops, arrange scallops in the skillet in a single layer and cook, without touching, until deep golden brown underneath, about 2 minutes. Flip and cook until deep golden brown on the second side and just firm and opaque, 1 to 2 minutes more.)

Using a slotted spoon, transfer scallops to a large bowl.

Leave any liquid in the skillet. Return the skillet to high heat and cook shrimp, stirring, until shrimp are just cooked through, about 3 minutes.

Using the slotted spoon, transfer to the bowl with scallops.

Return the skillet to high heat and combine calamari, garlic and chile flakes.

Cook until calamari are just cooked through, 2 to 3 minutes.

Scrape into the bowl with scallops and shrimp and let cool.

Add fennel, onion, parsley, lemon zest, lemon juice and remaining 2 tablespoons oil and 1/4 teaspoon salt to the bowl.

Toss to combine. Refrigerate for at least 30 minutes and up to 1 day.

Just before serving, taste and add more salt or lemon if needed.