I have a very distant cousin in Alsace and got the idea from her several years ago when I was doing some genealogical research. This recipe appears to be limited to the Rhine River (Fr and German side) with some places in France's Gold Coast offering it. I am not sure why it has not had success elsewhere.
Wonderful recipe - I checked some other recipes online after you posted yours, I’ve never seen that one before, and yours had the extra (and definitely smart) step of soaking the onions overnight.
The flavor! Wow - it didn’t even need bacon!
You would think to throw in some spices or something...but that would ruin it - it was perfect.