Ways to counter the burn of too many scallions?

Cauliflower fritters. Mix some egg, ice-cold water and flour, then stir in the other ingredients. Gently put about one or two tablespoons at a time into a deep-fryer using two spoons. We do something similar to make "kaki-age", which is a mix of vegetables prepared using a batter similar to tempura, and generally includes onion. You can probably also get away with doing this in a frying pan with just the bottom covered in oil if you keep the oil temperature stable enough.

For what it's worth, I prefer to make mashed potatoes by ricing them then adding butter, milk, scallions and cream then letting them roast for a while in an enameled cast-iron pot. I've never noticed an unpleasantly raw taste with the scallions in this way. Perhaps a similar approach would work for the cauliflower.

Can you get away with cooking the mashed cauliflower more? The only thing that really kills onion flavors is heat. Maybe you could turn the cauliflower puree into croquettes and fry them?

I don't think there's anything you can add that will neutralize the onion flavour itself. However, since the only other ingredient is cauliflower, you should be able to just pop it in the microwave for a few minutes. The onion will caramelize and turn sweet.

Of course this assumes that you haven't blended it into a pulp and released all of the sulfur compounds already. If you have, then they've basically contaminated the cauliflower and there's nothing you can do to remove the taste.

You could also try incorporating some parsley, mint, or cilantro, all of which "mask" the flavour of onion (but again, don't actually eliminate it).

I've also heard people say that lemon juice works for them, but that sounds illogical to me because the the sourness is already being caused by acidity due to the sulfur. Nevertheless, if the alternative is throwing it out, it's something you could try.