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.

Onions frequently suffer from onion maggots, which can travel easily from plant to plant when they’re spaced close together. Other onion-like plants, such as garlic, leeks, and shallots, are also common targets of onion maggots. Avoid planting them near onions so the onion maggots can’t travel easily.
Delft Perpetual Leek
A perennial leek that makes a cluster of new shoots around the mother plant. When the mother plant flowers and dies, the offshoots go on to make mother plants themselves. A real "permaculture leek" if harvested by carefully cutting the leek above the root or allowing unharvested leeks to flower before cutting back. Leeks are more disease free than onions, providing reliable winter food. Frank Morton's quick and careful eye spotted and selected these out of the heirloom "Delft" leek. A great new possibility for year-round gardens. Very rare. 20 seeds. $3.50 quailseeds
Leeks
For plants already grown: Plant out as soon as possible when conditions allow in a sunny spot where available. Prepare the soil well before planting, incorporate a good quality compost and water well. Prepare holes 4-6in (10-15cm) deep with a dibber and set the plants gently with out firming them in. Fill the hole with water, this will consolidate the plants sufficiently. Ensure the soil bed is well watered throughout the season and weed well.
Feeding Leeks
It pays to prepare the soil well before sowing leeks or adding young plants. In the winter add lots of bulky compost or manure to the soil to increase the nutrient levels. To top up and really put the young plants in good stead, add a general purpose fertiliser a couple of weeks before sowing or planting. To increase the thickness of stems feed regularly until mid-summer. If you prefer leek-whites to leek-greens earth soil up gently while they are growing, making sure you don’t get soil in between the leaves.
Watering Leeks
Water seed beds after sowing or containers if growing early crops indoors. When transplanting leeks into their final hole, fill the hole with water as this settles and firms the roots, while they thicken to fill the hole. Water leeks during dry weather. They are quite self-sufficient when they are not competing for water with surrounding weeds – so keep these to a minimum – or not exposed to long periods of dry weather. marshalls-seeds

The traditional method of planting leeks is to make a hole with a dibber, drop the leek into the hole and then ‘puddle in’ by gently filling the hole with water. Over time the soil will gradually backfill around the leek and blanch the shank (stem), making it paler and more tender. The only problem is that soil inevitably finds its way between the layers of leaves, making washing leeks hard work. Some gardeners use toilet roll tubes to blanch the shank instead, but I found that the space between tube and leek quickly became a popular hidey-hole for slugs, so this experiment was soon abandoned

loose soil allows the shanks to swell.

Spacing leeks at least 20cm (8in) each way will result in plumper plants, and also makes it easy to squeeze in a spot of intercropping. I like intercropping with lettuces as they fill the space between the upright leeks, helping to keep weeds down. As they’re short-lived plants they can be lifted before the leeks need the space to fatten up into.

It’s a good idea to mulch with compost between leeks to keep fertility up, weeds down, and moisture in. Leeks don’t need much additional watering unless it’s very dry. A liquid feed such as seaweed is beneficial a few times during the growing season. growveg
Onions tolerate soil pH as low as 5.5, making them a suitable crop for moderately acidic soil. Plant them in spring from sets for fastest growth. They need consistent water and full sun.