Not a USDA recommended recipe!!!!!
To caramelize the onions I used a crockpot set on low - one stick of butter, crockpot filled with sliced onions, cook for about 24 hours.
Allow the onions to drain (reserving the liquid) then pack in half pint jars. Pack the onions in the jars - added the reserved liquid as needed.
Clean the jar rim very carefully (paper towel soaked with alcohol) to remove any fat from the rim.
Can in a pressure canner at 10 PSI (for my elevation) for 40 minutes in half pint jars.
Note: I didn't add salt or pepper to the onions - you might want to add a little for the added flavor. These were red onions. You might want to run a test to see if you like the flavor. Use in salads, sandwiches, pizza, pasta dishes, quiche, bread recipes and much, much more.