a pun on the word 'Putonghua' (Mandarin); literally 'to boil a winter melon'
Definitions (粵典–words.hk)
名詞
普通話嘅官話諧音;略帶貶義 near homophone for "普通話" (Putonghua), i.e. Mandarin, the standard language of China; used for comedic effect or to convey a (self)mocking tone; literally: to boil winter melon
除非你去廣東或者廣西南寧嗰頭啫,上大陸好難唔講煲冬瓜㗎噃。
除非你去广东或者广西南宁𠮶头啫,上大陆好难唔讲煲冬瓜㗎噃。
ceoi4 fei1 nei5 heoi3 gwong2 dung1 waak6 ze2 gwong2 sai1 naam4 ning4 go2 tau4 ze1, soeng5 daai6 luk6 hou2 naan4 m4 gong2 bou1 dung1 gwaa1 gaa3 bo3. Unless you go to Guangdung or Nanning in Guangxi, it's hard not to speak in "poor tone hwa" when you visit mainland China.
Definitions (Wiktionary)
noun, Cantonese, humorous
Mandarin Chinese, especially when spoken with a heavy Cantonese accent