Prepare canner by inserting rack, and filling with enough warm water that it will reach 2” above canning jar lids when jars are in canner.
Prepare jars and lids by washing and/or sterilizing in hot water.
In a large bowl, combine squash and onions
In a large saucepan, bring remaining ingredients to a boil
Add squash/onions to boiling brine mixture, and ring back to a boil, cooking until vegetables are just slightly tender (this should take about 5 minutes)
Ladle into clean jars, leaving about 1/2 inch of headspace, and fit with lids and rings, tightening rings firmly, but not so tight that only Captain America can get them off. Just firmly enough so they’ll stay put till they seal.
Invert hot jars for 3-5 minutes, or
Place jars in canner, being sure they’re covered by at least an inch of water
Bring to a boil and process 10 minutes.
Remove canner lid and let cool an additional 10 minutes
Remove jars from canner, being sure not to expose them to cool drafts, which could break your jars. My mom taught us to grip them with a jar lifter with one hand, while covering the jar with a towel with the other to keep drafts away, and also protect yourself from any hot liquids dripping from the jar or if, heaven forbid, the jar should break.
Let the jars continue to cool, preferably covered with that towel until they’re no longer piping hot