For 6 lbs of onions, I use a 6-quart oval slow cooker. If your cooker is smaller, use fewer onions.
Peel and cut the onions in half, then slice them about ¼” thick. Cut the butter into eight pieces and scatter them over the bottom of the slow cooker insert.
Layer the sliced onions and fresh thyme leaves into the slow cooker. Don’t be afraid to completely fill the pot – they will cook down and not spill over.
Cover and cook on high for 2 hours. Stir the onions well, then replace the cover and cook on high at least 6 hours more, until the onions are tender.
When the onions are tender, if there is a lot of liquid in the pot, remove the lid and continue to cook on high another hour or so, until most of the liquid has evaporated.
Turn off the slow cooker and let the onions sit, uncovered, for an hour to cool.
Transfer to cup or pint jars and store in the refrigerator.