The dish was tolerably good, at least better than I expected.

tolerably 是副词，由形容词 tolerable（可忍受的、尚可的）加上副词后缀 -ly 构成。tolerably 在这里表示"还算可以、勉强过得去"的意思，带有"虽然不完美但可以接受"的意味。