Nowhere I’ve found so far anyway; you can seemingly find almost any vegetable you can name pickled and sold in a jar- but not diced onion. I’ve seen chopped onion, and dried minced onion is pretty common in the spice section. But if I want some diced onion on my hot dog I have to get out a whole onion and a knife. What gives?
I can find fresh diced onion in a small plastic tub in the produce aisle of the supermarket.
They have it at my grocery also. It doesn’t have a long shelf life.
It’s the Big Onion conspiracy. They have you now. I wish you well.
Moved to Cafe Society.
Colibri
General Questions Moderator
You just have to keep peeling it back, layer by layer.
There’s plenty of frozen diced onion, it’s in a ghetto part of the grocery freezer where there’s okra, chopped green pepper, black eyed peas, and other non-typical vegetables.
Dicing an onion is, what, the second-easiest thing in all of cooking? After boiling water?
And yet it’s not worth it if you’re only using a spoonful.
I buy mine from the ghetto frozen section too. I take out a bit and nuke it a sec, no muss no fuss, there ya have it. They also sell mirepoix with the celery and carrots and bags of mixed bell pepper and onion. I love to cook. I do it every day. But I really hate chopping smelly veggies.
Trader Joe’s does. They sell chopped onion in a bag. I can chop an onion with my eyes closed in a matter of seconds. Well not really with my eyes closed, but you know what I mean. I will never buy them.
It’s right next to the canned lettuce.
mmm
The supermarkets near my house sell diced onion. I use it frequently. It comes in a plastic tub.
Evidently not, as the plethora of “How to dice an onion” cooking guides and videos suggests. (I haven’t come across any comparable guides on how to boil water.)
Dicing an onion isn’t particularly difficult once you know how it should be done. But it’s still pretty labour-intensive, it can be unpleasant (strong vapours causing tears), and it requires a dedicated cutting surface. I think pre-diced onion would be a great time and labour saver for a lot of large, onion-heavy recipes. Unfortunately I’ve never seen it at any supermarket near me.
As far as onion goes, a fresh onion kept in a baggie (squeeze the air out before you seal it) will stay good in the fridge for a couple of weeks. A tablespoon here, a quarter cup there, and before you know it, you need another onion!
I should have known someone would come up with something like that. 
In case it’s not obvious, though, that tongue-in-cheek piece isn’t comparable to the many serious guides on onion dicing I was referencing.
I dice a half or quarter onion and ziplock the rest of the onion uncut.
I’ve tried to dice onions, after watching cooking shows and YouTube videos demonstrations. I can do OK, but can’t get really identical sized dices like the pre-diced onion. And it’s not really that expensive, given how infrequently I need it.
I take a whole unpeeled onion… or more likely a zip-lock bag from the fridge with an onion that already had a “spoonful” removed. Put the onion on a cutting board root side down, and take a Chef’s knife and cut one of the sides for as much or as little as you need. Put the onion back in the zip-lock. Pull off the peel and outer layer and toss in the trash. Chop, dice, mince… whatever you need. Takes seconds and costs a fraction of buying pre-processed.
I’ve never understood why people think this or peeling Garlic is difficult.
I use my mandoline to perfectly tiny-dice a small red onion and keep it in a small container in the fridge. It takes literally seconds, it’s good for like a week, and it’s pretty much a week’s worth of diced onions ready to go for anything (and I basically sprinkle some on everything).