For the most flavorful fish, gently poach the fillets in scallion oil, which is called pa gireum in Korean and is the star of this easy, foolproof preparation. Simmering scallions in olive oil over gentle heat removes moisture from the alliums, crisping them and concentrating their savoriness. In turn, the oil will be tinted green and perfumed with an umami-saturated scallion aroma like nothing else. Be sure to dip crusty bread into that glorious scallion oil to enjoy with the tender fish. This dish is great with rice, too.