18 hours ago, Toliver said:It was a sinus-clearing mustard dipping sauce using Colman's Dry Mustard (a British product, I believe) in the bright yellow can.
Many of the recipes I've seen for this mysterious "Chinese mustard" use Colman's Mustard. Yes. It is English.
My Chinese language food dictionary's entry for "mustard sauce" mentions that it was invented in Durham, England in 1729, then describes French and Italain mustard sauces. No mention of it being used in China.