3 to 5 pounds white or yellow onions (4 to 5 large onions)
3 tbsp extra-virgin olive oil or melted butter
1/2 tsp salt
Peel and thinly slice all of the onions into half-moons. Transfer all the onions to the slow cooker — the slow cooker should be half to three-quarters full.
Drizzle the olive oil or melted butter and the salt, if using, over the top of the onions. Toss to evenly coat all the onions with a thin glaze of oil.
Cover the slow cooker and cook for 10 hours on LOW. If you’re around while the onions are cooking, stir them occasionally — this will help them cook more evenly, but isn’t strictly necessary. After 10 hours, the onions will be golden-brown and soft, and they will have released a lot of liquid. If you like them as they are now, stop cooking and pack them up.
If you’d like jammier, more concentrated onions with a deeper color, continue cooking for another 3 to 5 hours on LOW. Leave the lid ajar so the liquid can evaporate. Check every hour and stop cooking whenever the onions look and taste good to you.
Remove the onions with a slotted spoon and transfer them to refrigerator or freezer containers. If any liquid remains in the slow cooker, transfer the liquid to a separate container — this can be used as cooking broth in another recipe. Onions will keep in the refrigerator for one week or in the freezer for at least 3 months.
If you’re planning to freeze a portion of your batch (and you should!), I recommend freezing them in containers of multiple sizes: ice cubes of caramelized onions are great for last-minute burger and sandwich toppings, cup-sized portions can be used for pizza and pasta toppings, and larger containers are perfect to throw directly in a soup.
A few notes about your onions:
- Onions with low moisture content are best. White onions tend to be drier and spicier and therefore an excellent choice to caramelize. When you slow cook them over an extended period, their natural sugars caramelize, and they have this intensely delicious flavor with a nice tender bite to them.
- Yellow onions often have a milder flavor than white onions. They still caramelize nicely and keep that tender texture. Try to choose large all-purpose yellow onions that are firmer.
- I would be more reluctant to choose sweet onions (like Spanish yellow onions) as they have a higher moisture content. Sometimes Spanish sweet onions can result in more loss of sugar and therefore less flavor with a more mushier texture. I would recommend using these types of onions more in their raw state like on salads instead of in this recipe.
- Red onions are definitely on the sweeter side than yellow or white onions. Be aware that they can turn out more grayish or with an almost brownish-burgundy color.