I do my own method, based on trial and error really:These look great. May I ask which recipe you used? I made my first ever pickled shallots and onions November last year so they would be ready for Christmas. My recipe didn’t ask for any sugar and as a novice, I went ahead and was very pleased with my efforts and looking forward to Boxing Day buffet with a few pickled shallots on the side. They were, to put it mildly, very strong, in fact, my father in law said very loudly STREUTH! After his first one! I’ve since added a few spoonfuls of sugar to each jar and they have calmed down a bit but they are now 7 months mature. By the time this Christmas comes around they are going to be corkers!!
Salt the shallots or onions (liberally) overnight, next day drain and rinse.
Heat malt vinegar slowly in a pan with chosen spices, then add brown sugar to taste, and I do taste the vinegar until I think it's balanced enough.
Let vinegar cool to at least lukewarm, if onions go in with hot vinegar they will start softening.
Pop vinegar and onions in sterilised jars, I usually add bay leaves too and sometimes pick one jar to have some hot chillies added
They come out really crunchy using this method. I too in the past have made the no sugar error