Kaity - I would NOT plant them with aspargus. Aspargus needs room to grow and spread it's roots. Planting and pulling onions would just disturb them.

You can plant onions 1/2 to 1" deep. Plant them 4" apart if you're going to leave them to "bulb", every 2" if you're going to pull every other one to use as green onions. If you leave them at 4" spacing, just water...and watch. IF they shoot up a stalk and bloom, pull them. That's what they call "bolting" and that onion will never be any bigger or better than what it is when that happens. Otherwise, they'll break over on their own when they're ready to pull.

Others that are smarter than me may come along with better and more detailed advice, but that's the best "short story" I can offer.

Paula

Even if you didn't buy from Dixondale, this is still a good read. You do mean plants, right?

Here is a link that might be useful: Planting Guide