I've noticed that raw onions or shallots taste much more bitter when they are blended in a food processor than when chopped with a knife. Why is that?

onions are mostly water and the finer they are chopped - the more water there is -
a food processor does not do a good job on chopping onions in uniform pieces (some are tiny and some are large) and when cooking, the smaller pieces cook first and turn bitter before the larger pieces of onion have cooked

onion pieces need to be of uniform size to cook properly - it's best to hand cut them - and high heat should never be used to cook onions - high heat causes bitterness -

cook onions of uniform size low and slow for best results

How about raw onions? I mean when there's no cooking involved, the processed onions will taste bitter, while the sliced onions will not. I wonder if there is a way to finely chop onions with a machine or something more sophisticated than a knife without causing them to taste bitter when eaten raw.

here's a thought . . .

finely minced onions - like some hot dog vendors make - seem to oxidize and go off flavor much more quickly than sliced or even large diced. not sure I would call it bitter - yuukky perhaps . . . I always sniff a spoonful before proceeding to dress my dog.

I suspect the fine dicing releases so much of the onion liquid that the liquid may actually be the component oxidizing and producing the funny flavor.

I won't use minced onions on my hotdogs - it's just plain crappy! My onions (and I load them on hotdogs!) are cut in large pieces. Fine mincing can be poisonous - to me at least.

Stick to the bigger dice.