re: canning onion aauce

You need to follow a safety tested recipe when canning any food. Otherwise you don't know the ph nor the density. Both factors are very important in home canning. The recipes in the Ball Blue book or any of the USDA/extension recipes will be safety tested and good to use.
If you make up your own recipe or use another one from some source that has not been tested in a lab, you could get food borne illness, the most serious and possibly deadly, botulism.
As for the jars, they are not recommended for home canning. They are very hard to tell if you have a seal or not. Only the standard 2 piece jars and lids are USDA recommended. We tell people to embelish your jars with nice labels instead.
It would depend upon the recipe you use as to whether or not it needs pressure canning or not. The safe recipe will tell you that information.