-Beets Plant near: broccoli, brussels sprouts, bush beans, cabbage, cauliflower, chard, kohlrabi, onions Keep away from: charlock, field mustard, pole beans
Lettuce Plant near: beets, carrots, parsnips, radishes, strawberries Keep away from: cabbage family Comments: lettuce tenderizes summer radishes
Onions Plant near: beets, cabbage family, carrots, chamomile, lettuce, parsnips Keep away from: beans, peas Comments: deters most pests, especially maggots.
-Broccoli and Brussels Sprouts Plant near: beets, buckwheat, calendula, carrots, chamomile, dill, hyssop/anise, marigolds, mints, nasturtiums, onions, rosemary, sage, thyme, wormwood. Keep away from: strawberries Comments: marigolds repel cabbage moths. Nasturtiums repel aphids

Beets thrive when planted near bush beans, soybeans, butter beans, cauliflower, broccoli, kale, peas, Brussels sprouts, kohlrabi, leeks, scallions, onions, garlic, mint, summer savory, borage, and Swiss chard.
Beets are also compatible with celery and lettuce. All varieties of beets and Swiss chard will cross-pollinate with one another.

Taproot and Fibrous Root
In order to increase crop production in a limited garden space, savvy homesteaders may plant two compatible vegetables that use different layers of the soil profile together. Examples include fibrous-rooted tomatoes with beets and other tap-rooted vegetables such as carrots, turnips, and parsnips. -source

Radishes are another useful companion plant for beets. Gardener Marie Ianotti says that “You can always use the old trick of planting fast sprouting radishes in the same row as your beets. It helps mark the row and loosens the soil. By the time the beets start to develop, the radishes are ready to be pulled.”

Best onion plant companions: cabbage family; Broccoli Kale Brussels sprouts Cabbage. This is because onions naturally repel pests that love cabbage family plants, such as cabbage loopers, cabbage worms, and cabbage maggots.
Onion plant companions are:
Tomatoes
Lettuce
Strawberries
Peppers
Bad Companion Plants for Onions
All varieties of peas and beans can be detrimental to onions. The same goes for sage and asparagus.

Nitrogen-fixing, pairs well with squash and heavy feeders. Keep away from allium family.

-Radishes Plant near: chervil, cucumbers, lettuce, melons, peas, nasturtiums, root crops. Keep away from: hyssop/anise. Comments: radishes deter cucumber beetles. Lettuce helps make radishes tender. Nasturtiums improve radishes' flavor.
-Carrots Plant near: Peppers, cabbage, chives, leeks, lettuce, onions, radishes, rosemary, sage, salsify. Comments: onions, leeks repel carrot flies
-Lettuce Plant near: beets, carrots, radishes, strawberries. Keep away from: cabbage family. Comments: lettuce tenderizes summer radishes
-Squash – Companions: corn, lettuce, melons, peas, and radish. Avoid planting near Brassicas. Radishes deter cucumber beetles
Carrots, cucumbers, radishes, squash, and members of the Allium family all do well when grown in close proximity to peppers.
flea beetles and root maggot flies