Depending on the quality, Black-Back Perch Stew restores 26/28/30% of Max HP and regenerates 450/620/790 HP every 5 seconds for 30 seconds for the selected character. Like most foods, this has no effect for other players in Co-Op Mode.
According to Zhongli in the Stone Harbor Treasure Journal event, using Violetgrass in Black-Back Perch Stew is the Wanmin Restaurant's secret touch. Based on Chef Mao's baffled reaction in the background, Xiangling was the one who came up with the idea.
This dish is likely based on shuǐzhǔ-yú (Chinese: 水煮鱼 lit. "water boiled fish"), a variant of the Sichuan dish shuǐzhǔ-ròupiàn (Chinese: 水煮肉片 lit. "water boiled pork slices").