I find the easiest time to start the onions in right before you go to bed, that way they can cook all night undisturbed
First you thinly slice 5-8 large white onions, depending on the size of your crock pot, and toss with 3-4 Tbsp. melted butter and ½ tsp. salt, making sure all the onions are lightly coated.
Then you put the lid on, turn the crock pot on low, and go to bed! In the morning, they will be a light golden brown and there will be a lot of liquid in the pot.
You can call it quits at this point, but I like a really jammy concentrated flavor out of my caramelized onions, and if that’s what you are looking for, plan on another 8-10 hours of cooking. At this point you will need to prop the lid of your crock pot open so that all the liquid can slowly evaporate. Keep the onions on the low setting and stir once every couple of hours until most of the liquid is gone and your onions are a dark brown. It always surprises me at how much the onions reduce in size, but you will still have way more onions than you can cook in a pan on the stove top.
The onions freeze well, and one of my favorite ways to store them is to portion them out into ice cube trays, freeze and then pop out into a freezer Ziplock bag. They will keep for up to 3 months, and a single cube is the perfect amount for a sandwich.