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Bistro 25 in Sayville is a real winner (try the lamb porterhouse). For sushi, there's Onsen in Oakdale. And I really like Thai Angel in Islandia.
If you don't have a car and you're staying at the Hilton Garden Inn, Mama Mia's is walking distance and not bad.
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Thai Angel Restaurant
1812 Veterans Hwy, Islandia, NY 11749