About 4-5 years ago I attempted to preserve some lemons in salt as per lots of recipes for Morrocan preserved lemons for tagine etc. I didn't ever get around to using any of them and also used an inappropriate clip top jar which allowed gas to escape and the salt badly corroded the metal parts. The lemons turned an unappealing brown colour and also went gross & mushy so I finally tipped them out into the compost today. The weird thing was the salty juice which seeped out of the lemons had turned into a really well set gel. Everywhere I have looked claims that pectin only sets with sugar and jam also needs cooking to 105 degrees c otherwise no gel will form. How did these raw salted lemons make a gel? It would be very cool if I could do the same thing with sugar and make a raw citrus marmalade! 
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. 