Hoping someone can advise, on a whim I thought I'd try and pickle a couple of jars of shallots. I used the same recipe as for onions malt vinegar (5%), salt shallots over night (dry), spice vinegar - let that go cold and top up a couple of jars to the brim, shallots covered. Checked this morning after four days and one jar has a fair amount of gas - looks like fermentation, the other has some activity, possibly air bubbles (?) Haven't seen this before, I'm assuming that this isn't normal (and jars should be discarded) as I've used vinegar ?  Ingredients and method from https://www.sarsons.co.uk/recipes/pickled-onions :  
In general, when using a thermal cooker, the basic procedure is this:  Start some water boiling Bring your soup base to a boil (separately) Pour the boiled water into your insulated flask Make your soup and let it boil for a few minutes (fully cook any meat, typically, but not more than 5 minutes) Dump the water out of your flask (it was just to pre-heat it) Pour the soup into the flask with any items that shouldn’t be boiled and seal it Wait a few hours Eat  You can find recipes by searching for ‘thermal cooker recipes’, but some are for appliances which have their own cooking element in them, so don’t require the pre-heating step.  You can also try searching for ‘soup bento’ recipes to try to find something similar to base your initial ratios on, and then adjust from there to your taste and time delay. 