Put the sliced onions in the slow cooker. Top with the butter.
Set the slow cooker to low and cover the cooker.
After nine hours, stir the onions and continue to cook them an another 12 to 15 hours, occaisonally checking and stirring.
They will be a deep, dark brown when they are finsihed.
Leave the top off and continue to cook for another 2 to 3 hours, or until most of the liquid is gone.
Use the caramelized onions in soup, on burgers, etc. Store the remaining onions in the refrigerator or freeze them.
Notes
Four large onions, about 4 quarts sliced, will yield about 3 cups of caramelized onions. For Vegan Caramelized Onions, omit the butter and substitute a tablespoon of flavorless oil.
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