Peel the onion, slice it into ¼-inch thick slices, and separate the slices into rings.
Spread the onion rings in a glass baking dish. Add the olive oil and toss to coat. Sprinkle the onion rings with salt.
Roast the onions, stirring once halfway through roasting, until soft and caramelized, about 20 minutes.
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Notes
How do you slice an onion? You wash, dry, and peel it. Then slice it into ¼-inch thick slices and separate the slices into rings. You can also cut them into quarters if you prefer and roast them for a little longer.