According to my trusty farmer's almanac here, onions are pretty much a friend of anyone, with a few exceptions. It says they work well with beets, ratishes, and tomatoes, and are of benefit to cabbage and carrots. But they don't do well beside beans.

I am actually planting my onions right next to both tomatoes and spinach this year.

As far as spacing, my seed packets say to space spinach 6" apart. The tomatoes say 3-4' spacing. So it sounds pretty good the way you have it. I just planted about 60 Georgia Sweet transplants today. What kind are you growing?

Kim

Onions only require a small spacing, as dictated by their size.
Say, your onions will be 3 inches of diameter. That is all they need.
Actually their roots need much less in-groung space because
they go vertically down. So 10" space shouldbe more than enough.

first timer with onions..the package said to space them 12 inchs in between each row. Is that overkill? Can I fit in another row? Right now I have two rows of onions with 12 inch in between them.

Matthew I plant in a block 4-6 inches of space in all directions around each one

4-6..reallly?? The package called for 12. Wow..maybe I put another row in bewteen.

If you planting in blocks, 4-6 inches each way works fine in enhanced soil. ( depends somewhat on the size onion you are growing, small bulbs closer than Jumbo types). If you are planting a field of onions, 12 inch row spacing is about the minimum you can get away with.
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With great respect to mmaster Farmerdilla, I think the purpose of RO0W spacing in a field iS to provide access, to weed,
feed, walk etc. But when one is planting a few in a small home garden
he/she does not need that mucH space, especially if land resource
is at premium. This is where squrae foot gardening comes to play.
It makes an efficient use of garden space. Building raised beds are
costly. One cannot afford to leave 12 inches between onions.
But a farmer with acres under planting, using machinary to plant, cultivate an harvest
cannot get away with less than 12" ROW SPACING.

EXACTLY cyrus. Blocks are appropriate for small gardens, where one can reach across the block to weed. Rows are for production growers where one must physically intrude into the growing area whether it be a quarter acre or a thousand acreas.

If you're growing onions, plant about three times as many as you plan to harvest for the fall. I do mine about an inch and a half apart. This year, I used an entire bag of 80 onion sets to fill one 5'x1' row. Throughout the summer, as they get bigger, I will pull up most of them as green onions, eventually leaving about 3" between the remaining plants. That way, I get green onions all summer, and towards the end of the summer, some small onions to cook with, and in the fall, big onions to keep around, all out of just a small garden space. I will also plant summer savory and chamomile in the space around the onions when it warms up, thus preventing weed intrusion and also giving me a supply of culinary herbs and flowers for chamomile tea. The chamomile flowers will also be great for attracting pollinators for my nearby fruiting plants.

If you're in a small garden, it's vital to use every square inch of space. I have a couple foot-wide rows of allia (two varieties of garlic, two varieties of onions, and winter or walking onions) and in each row I've planted about five or six wide; at the end it will be about three plants wide. In between these rows of allia, I'm putting in carrots, beets, spinach, chard, etc.

Always grow some herbs in among your plants! Weeds like to grow up in your garden space anyway, you might as well give the soil and light space to something you can use!

I like to do four rows at a time one foot apart. I can comfortably do two rows one direction and the two rows the other. That leaves a 2 1/2 foot path then between the beds. For larger onions like Candy or Kelsae, I space them 6 inches apart in the rows--4 for all others.