Mostly, I make use of space . . . By that I mean, there is almost always room for an onion set, somewhere.
I don't think that onions, actually a heavy feeder, appreciates
crowding anymore than any other plant. Still, if you are needing a scallion that is less than 1/2 inch in diameter, it doesn't need a square foot of soil to grow in.
Leaf lettuce also
fits in well. Lettuce can tolerate some shade. It also benefits the gardener to protect lettuce a little from a blazing hot afternoon sun, especially in a dry climate like where I garden. Leaf lettuce isn't destined to stay in the garden for long, either. Growing along the outside of a bed of carrots works just fine for lettuce.
Many of my vegetables are planted in beds and because I grow a lot, they sometimes take up an entire bed by themselves. But if I plant only a few square feet of something, it seems to go well to work it into a bed with something else. Often, I have "salad beds" of all sorts of things.
I read
How to Grow More Vegetables, by John Jeavons, many years ago. Kentucky State University uses some of his diagrams to give you ideas about spacing when companion planting.
KSU - click here.
This is a real good article about
Companion Planting. The author is a knowledgeable
advocate of the practice. But, I want you to note what she says about this weird idea of "crystal chromatography" upon which so much has been built regarding companion planting -- plants actually "liking" or even "loving" other plants.
Well maybe, but I think there's reason to believe in some "benefit" and a little "tolerance" on one plant's part towards another . . .
Steve
, the b&w hexagonal illustraion looks like a diagram of a chemical compound!! 
--every little bit of information helps.
