Caramelizing onions is a lot of work. This method makes it very easy, so almost all of the work is in the prep.
Slice onions thinly. Use a mandolin, if you have one. Slice as many onions as you want. Yellow, white or red, but don’t use sweet onions. I think a 5 lb bag works great, and it isn’t really worth the effort to make a small amount. Add them to a microwave safe bowl. Try not to cry.
Microwave onions for 15 minutes. I have to do this in multiple batches, but you can do it in 1 if your bowl and microwave are large enough, though adjust cooking time accordingly. Listen to complaints about the smell.
Strain liquid from onions. Poke them a bit with a spoon or tongs to get all of the water out. Move the onions to your slow cooker with a bit of olive oil – about a tablespoon.
Cook on low temperature for 12 hours or high for 6 hours. Do not lift the lid while cooking.
Stir the onions up a bit and move the lid so that is not fully covering the slow cooker.
Cook on high for 6 hours. Feel free to stir the onions now, but it isn’t necessary.
Keep going if you are looking for a denser texture. Try a 1/2 hour at a time.