Another vote for shallots, although that’s in no small part due to my growing them. There are a few varieties of shallots out there, but they aren’t as easy as onions to find. I keep leftover pieces of 1/2 white, red, and yellow onions as well, and usually end up using them in some recipe within a week ( I make the serious eats puffy taco filling weekly), but will toss them if they aren’t “right” .
I love an old article from Saveur about alliums. I think this is it.
Many have suggested shallots, but can you buy a single shallot? I only see them in bags of a dozen or so, probably much more costly and wasteful than a single RO? Wrapping half an onion tightly in clingfilm (sarin?) gives it a fridge life of about 5 days, I find.
IMO dried onions are a completely different animal, and unless you need their particular presence (onion soup, maybe?) you would be better just to omit onion from the recipe.
You can buy them as singles most of time here in N Cal, especially in Asian markets. They come by the 1/2 pound bag from my CSA, and if they are okay when you get them (no mold), they seem to keep at room temp or in the fridge for awhile. I don’t keep them in the fridge, but I’ve read about it.
I also compost a LOT of “not right” veg.
I did just the other day, some stores have them loose by the pound
Grate leftover onions with garlic. Add to (lamb) mince with parsley and (Middle Eastern) spices. Form balls or sausages and you have köfte/koftas.
Or add them to stir-fries, sauces. Onions and garlic are the basic flavour builder of so many dishes in many cuisines.
In addition to what others say about substituting shallots, I often just use the whole (small to medium sized) red onion in a dish if doing so wouldn’t throw the recipe out of proportion. We like onions and I find that extra onion in cooked dishes can be a happy thing. In longer cooked dishes I find that onion melts away.
I stick with the recipe’s recommended amount if the onion is to be left uncooked because raw ones pack a punch.
Freeze the leftover is you won’t use within a week. Texture won’t be crisp but flavor is okay. Or buy granulated onion. In such small quantities red vs yellow onion isn’t a big deal.
When I need a small amount of produce I often check the salad bar if the store has one. While it’s much more expensive pound for pound, you may be able to purchase the amount you need.
Good thought on the salad bar!
(I appreciate all the suggestions, but many people missed the part where I said I don’t cook very much. Basically, if I cook once a week, that’s about it. Pickling the remainder seems doable too.)
I’ll dig out the red onion pickle that I really liked. I was tossing it in everything!
Super easy:
Nobody likes waste, but pickling half a red onion? The toss it option looks good to me.
I get single shallots at several supermarkets in the Boston area. And for @eleeper, if you use a Ziploc vacuum seal bag, the red onion can stay fresh for a long time in the fridge after chopping off what you need. I don’t have the electric sealer, but the hand pump, and it works quite well (although I couldn’t find the bags anymore in the supermarket, so bought a large batch from Amazon).
An alternative is to caramelize the rest of the red onion and freeze in Tbsp. quantities on parchment paper, then peel them off and pop them into a freezer bag for use as you need them.
The salad bar can be quite convenient. I often use it when a recipe calls for one celery stalk as no one eats celery in my household.
Celery Nation is not happy .
My family swears they are going to eat and it always ends up a limp mess.
I’m a heavy shallot user and I agree. I’d use a cipollini onion instead - they are sweet and mild like red onions and more to the point they are tiny. Most places I go for vegetables sell everything loose so you can easily buy one shallot or one cipollini onion.
Great idea! I 
cipollini onions but find them SO much harder to find or grow than red, yellow, or hybrid shallots (haven’t tried to grow French grey). Not giving up though, especially after this!
My usual soup recipe starts out with celery, onions, and carrots, sauteed for a while before adding liquids or whatever kind of soup material it’s going to be. Recently my refrigerator has been randomly deciding to freeze the vegetable drawer, so suddenly it’s soup time; celery does ok if it’s been frozen and hasn’t re-thawed and turned into mush. The onions weren’t really frozen, but they’d probably do fine as well.
Soup is one of my favorite food categories. I try to make a pot every week.
