Marinate squid and tiger prawns in 2 teaspoons olive oil, chilli flakes and garlic. Keep, covered, in the refrigerator for at least 30 minutes and up to 4 hours.
Heat the remaining 2 teaspoons olive oil in a large pan over medium-high heat. Add leeks and asparagus and cook until tender, 3 to 5 minutes. Toss in the squid, prawns, smoked trout, tarragon, basil, spring onions and lemon rind; season with salt and pepper. Cook and stir until the seafood is cooked through, about 5 minutes. Serve immediately with lemon juice sprinkled on top.