while in Hong Kong to host a series of April pop-up meals at local restaurant Lily & Bloom. 是省略了主语和be的时间状语从句。不定式短语是从句中在香港的目的状语。补全从句则为：

The restaurateur discovered the traditional Chinese rice porridge dish while he was in Hong Kong to host a series of April pop-up meals at local restaurant Lily & Bloom. 划线部分是从句的目的状语。