What to Do With Half an Onion (and Why It Keeps Happening)
Every recipe needs half an onion. Onions come whole.
So there it is again: half an onion, sitting on the counter or wrapped in something optimistic, waiting for a plan. If you cook for one, this happens constantly. Here's what to do with half an onion right now, how to store it so it's still good in a few days, and the simple change that stops it happening every single week.
How to store half an onion (cut side down, airtight)
A cut onion wants two things: to stay moist on the cut face and to stop perfuming your whole fridge. Here's how to give it both:
- Leave the skin on the uncut side. It protects the rest of the onion.
- Wrap it tightly or seal it airtight. A small lidded container or a zip-top bag with the air pressed out works best. Plastic wrap pressed right onto the cut face is fine too.
- Put it in the fridge, not back in the pantry. Once an onion is cut, it belongs in the cold.
- Keep it away from strongly absorbent foods like butter and soft cheese if it isn't sealed well. They'll pick up the smell.
If the cut face looks a little dry when you come back to it, just slice off a thin layer and use the rest.
How long it lasts in the fridge
Stored airtight in the fridge, a cut onion keeps for up to about a week. Use your senses: if it turns slimy, grows mold or smells sour rather than oniony, throw it out.
Chopped or sliced onion is also best used within about a week, and it loses its crunch sooner.
By contrast, a whole uncut onion keeps for weeks in a cool, dark, dry place with some airflow. That's why the whole onion was never the problem. Cutting it starts the clock.
5 ways to use the other half
Half a medium onion is about half a cup chopped. That's about right for most dinners for one. Pick whichever sounds good tonight:
1. Fried rice
Dice the half onion and fry it in a little oil until soft. Add leftover cooked rice, a handful of frozen peas, soy sauce, and push it to one side to scramble an egg. Ten minutes, one pan, and it's a great use for day-old rice too.
2. A two-egg frittata or omelette
Slice the onion thinly and cook it slowly in butter until golden, about eight to ten minutes. Add two or three beaten eggs, any cheese you have, and whatever greens need using. Finish under the broiler or with a lid on.
3. Quick tomato pasta sauce
Soften the diced onion with a sliced garlic clove, then add half a can of chopped tomatoes and a pinch of salt. Simmer ten minutes while the pasta cooks. (Yes, now you have half a can of tomatoes. Freeze it, or see the real fix below.)
4. Quesadillas
Slice the onion and cook it with a sliced pepper until soft. Pile onto a tortilla with cheese and any leftover chicken or beans, fold, and crisp in a dry pan on both sides.
5. Quick-pickled onion
Not a dinner, but it makes lots of dinners better. Slice the onion thin, put it in a jar, and cover with a mix of vinegar and water (about half and half) with a pinch of salt and sugar. After 30 minutes it's ready. It keeps for up to about two weeks in a sealed jar in the fridge and goes on tacos, rice bowls, sandwiches and salads.
Caramelize it once, use it all week
If you've got more than half an onion to deal with, caramelize it. Slice thinly and cook slowly in a little oil or butter over low heat, stirring now and then, for 30 to 45 minutes until deep golden and jammy. It cooks down a lot, so a whole onion makes a small jar.
Caramelized onions keep in a sealed container in the fridge for three to four days. Use them:
- On toast with cheese
- Stirred into pasta or rice
- On burgers, sandwiches or pizza
- Folded into eggs
They also freeze well. Spoon them into an ice cube tray, freeze, then pop the cubes into a bag. Each cube is a small flavor boost for a future dinner.
The real fix: plan dinners that share the onion
All of that helps once half an onion already exists. But the reason it keeps happening is that each dinner is planned on its own. Monday's recipe needs half an onion, so you buy one, and nobody tells Thursday.
The fix is to plan the week's dinners together, so the second half of the onion already has a job before you cut it. If Monday's chili uses half, choose a Wednesday or Thursday dinner that uses the other half: fried rice, a curry, a frittata. Then the onion isn't a leftover. It's two dinners.
The same trick works for everything that comes in awkward sizes: bags of spinach, bunches of herbs, cans of tomatoes, packs of chicken. Plan the second use at the same time as the first.
That's the whole idea behind 3Onions. It plans dinners that share ingredients across your week, so the other half of the onion is already in Thursday's dinner, and the grocery list notes show what gets used up. Join the 3Onions early-access list (the iPhone app is almost here, and your first week will be free), or read more about cooking for one.
FAQ
Can you freeze chopped onion?
Yes. Chop it, spread it on a tray to freeze, then move it to a sealed bag so the pieces don't clump. It's best used for cooking, not raw, because freezing softens the texture. Frozen chopped onion keeps its quality for several months and can go straight into the pan.
Is it safe to eat a cut onion left out overnight?
It's safest not to. Cut onions, like other cut vegetables, should go in the fridge. If a cut onion has been left out at room temperature for more than two hours, the usual food safety advice is to throw it out.
Does half an onion make my fridge smell?
It can, if it's not sealed. Use an airtight container or a well-sealed bag and the smell stays put. A container with a proper lid works better than foil or a loose wrap.