Thanks to tintin (yeah, farny name I know), a resident on mainland of Penang, we were brought to Khuntai Restaurant in Raja Uda, Butterworth, Penang. A spacious restaurant, with its own parking lot by the side (but you can actually park haphazardly anywhere at all, given the rather obscure location), this al fresco style restaurant purpotedly serves delicious yet authentic Thai food, at a fraction of the prices in higher end restaurants.
Geez, my memory failed me yet again. Price-wise, and species-wise. But this should be a siakap, steamed with the signature spicy and tangy sauce, and served over the fire. Firm and tender flesh, the fish was thankfully fresh and devoid of unpleasant fishy/muddy taste.