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1
Toast coriander, cardamom, and curry paste in a medium saucepan over medium heat, stirring, until spices are fragrant and curry paste begins to caramelize, about 2 minutes. Add cilantro stems, kaffir lime leaves, fish sauce, and 1 cup water. Bring to a boil, reduce heat, and simmer gently until flavors meld, 8-10 minutes.
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2
Strain broth through a fine-mesh sieve into a large heatproof measuring cup; discard solids. Let cool, then stir in lime juice; season with kosher salt and more lime juice, if desired.
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3
Meanwhile, using a sharp knife, cut all peel and white pith from grapefruit; discard. Working over a small bowl, cut between membranes to release segments into bowl; discard membranes. Repeat with lime.
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4
Heat vegetable oil in a large cast-iron or other heavy skillet over medium-high heat. Season fish with salt and pepper. Cook fillets, skin side down, 1 minute, then firmly press down with a fish spatula to ensure even browning and crispiness. Continue cooking until skin is golden brown and fish is mostly cooked through, about 4 minutes; carefully turn fish and saute until just cooked through, about 1 minute longer.
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5
Divide lime broth and fish among shallow bowls. Scatter citrus segments, cilantro leaves, and mint and basil over; drizzle with olive oil and season with sea salt.
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6
DO AHEAD:
