English mustard vs. "german" mustard
The German „Mittelscharfer Senf“ ist pretty wimpy compared to the average English mustard that looks deceptively similar.
You need something that packs more punch, if you can’t get proper English mustard (the Coleman’s in the other answer is occasionally available in German stores), a Dijon mustard (Maille is a commonly seen brand) will do, or a „Scharfer Senf“ (strong/sharp mustard) of a German brand.