Plain Greek yogurt with finely chopped green onions dip

I regularly store chopped onion in my refrigerator (or at least halves & quarters).

I either use tight-sealing plastic containers or zip-top bags. You may want to double-bag in zip-tops to be sure to avoid a smell.

One problem you may be having is onion-ness getting on the outside of the container. Be sure the outside is all clean and dry - no point in having a nicely sealed packet of onion when the outside can get all stinky anyway.

Escallion = scallion = green onion.

I think it's possible to substitute regular onions in some situations, but there are a lot of variables.

First off, scallions have a sharper, more grassy flavor than onions, though they're not as strong.

Then there's the question of which kind of onion you want to substitute. White onions have some of that same sharp flavor, and are quite strong. Yellow onions are much milder and sweeter, and might not be such a close flavor match. Red onions are somewhere in between, in my experience. I would probably try white onion, but in much smaller quantity than called for of scallions.

It's also important to consider how the scallions are being used. If they're being cooked into the recipe, that's very different than if they're being added right at the end, or even as a garnish. I think the substitution is less of an issue if the scallions are to be cooked. If they're to be more or less raw, I'd suggest white onion, not too much of it, and sliced very very thinly, as a raw white onion is really strong.

Whatever you try, bear in mind that it won't be exactly the same. But that certainly doesn't mean it'll be bad--just different.