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Besides, can you use onions that have been frozen?
To freeze chopped onions, wash bulbs well and chop as fine as you like. Thawed onions tend to lose their shape, so if you chop pieces super fine in a food processor, your thawed product may resemble mush. To use frozen onions, toss what you need into your dish and let them thaw as they cook.
Similarly, you may ask, why won't my onions caramelize?
You're not sautéing your onions—you're trying to slowly coax flavor out of them. It takes time, probably a solid 45 minutes, for the onions' sugars to caramelize. If your heat is too high, the onions will burn. Heat your pan over medium-low, then add your onions.
Make sure whatever pot or pan you're using has a lid. Turn the heat to medium and add the butter/oil and salt. When the onions start making those wonderful sizzling noises, give the onions a good stir, reduce the heat to low, and cover the pot/pan. After 20 minutes, check the onions.