I go to a food cart regularly and I've had this red lentil soup before but today I noticed that the minced onions were really crunchy but didn't taste strong as if they were raw. The lentils were soft so I'm sure that the soup was cooked long enough to make the added vegetables tender. I'm assuming this is because they weren't sauteed before they were added to the soup? Could there be some other reason?
Onions that have been blanched (briefly dipped in boiling water) will taste less sharp than raw onions - so it is possible that the onions were merely added at the end, and the soup may have been hot enough to mellow the taste even if the onions remain firm.
It may also be possible that the onions in the soup that day were dehydrated - if they had run out of onions and were substituting, or had dehydrated on hand because they are much more shelf stable, or they were just experimenting. Dried onions rehydrated in soup might have a crunchy or crisp texture like raw onion, without the sharp flavor raw onion have. Also, a dried and rehydrated onion might be crunchy even after being sauteed, and may not soften even when simmered, which could account for your soup.