Remove outer skin from onions and slice in half.  Then cut half moon slices about 1/8" thick

Rub the inside of the slow cooker crock with canola or olive oil to prevent sticking.  Place onions in slow cooker, add 3 Tablespoons olive oil and mix well.  Add Kosher salt, mix well

Replace the lid and cook on low for 8 hours.  You can keep this super simple and return 8 hours later, sometimes the onions will overcook and stick to the bottom of the crock. 

I find that a little extra effort yields superior results.  Every couple of hours remove the lid to relieve steam and dump excess moisture from the lid in the sink, at this time stir well, bringing the onions at the bottom of the crock to the top.

Test your onions after 8 hours, for some this will be enough sweetness.  For die hards like me an additional few hours is necessary.  Once again, remove the lid and excess moisture from the lid, stir the onions.  

Replace the lid but this time keep it askew, about 1" off kilter.  Continue to stir the onions every couple of hours until you have reached the dark color and degree of sweetness that you like.  I typically go for a dark brown glossy color and rich mellow taste.