Here the tuna marinates while you simmer up a vibrant curry sauce, complex in flavor but simple in every other way. Use any curry paste, from green to yellow to mussamun. The sauce is also terrific with grilled shrimp, roast chicken, or new potatoes and peas. Stir the canned coconut milk well before measuring it out, as it naturally separates as it stands. You can make the sauce ahead and reheat it gently close to serving time.