I start onions from seed, sets, and seedlings. I diversify so that if one method temporarily fails, I have the other methods to get onions from. That might sound crazy to most gardeners, but here in the tropics, crop failures are common. By using a variety ways of doing things, I'm more apt to have steady food to put on the table. After all, my goal is to harvest food, not adhere to the "best" way of doing it.
Seeds --- I find that seeds are by far the slowest way to produce onions. In addition, they take a lot more care and attention. Initially I had a lot of problems trying to get seedlings successfully. I'd sow a pack of seeds and be lucky to get a dozen onion plants. I quickly saw that direct sowing out in the garden was a complete failure and waste of effort. Poor germination. Poor survival. And if the turkeys found the patch, they'd eat them all. Fencing off the turkeys wasn't going to improve the situation much, because I got very few surviving seedlings anyway. Ok--- back up---try something else.
Now I start the seeds in a small tray, then as they sprout I transplant them into a larger growing tray, spacing them about an inch apart to give them room. The room just makes it easier to pull them for transplanting later. They will grow in one of the mini greenhouses until they are 1/8 to 1/4 inch thick, whereupon I transplant them out into a garden bed. Why bother with this? Because sowing directly out in the garden simply doesn't work for me, so I now use the mini greenhouses to produce my own transplants. And I like the idea of growing onions from seed I sow myself.
Sets --- In the past I've been given onion sets. They are an easy, though more expensive way to grow onions. Most of the onion sets for sale are long day types, so they don't bulb here in Hawaii. But they can produce thick shanks, looking like leeks, that are juicy and delicious. Sometimes the variety is an intermediate type and I get thick shanks with a wide bulby base. That's fine too.
Our local Wal-Mart usually has onion sets for sale each spring. And it's not unusual for friends to gift me a bag. I've always been successful with them, especially in soil enhanced with a generous amount of compost.
Seedlings/transplants --- I buy seedlings from Dixondale. I've always had great success with them. I usually order 10 bunches which gives me an abundance of onions. The biggest plantlets go right out into the garden beds immediately. Anything smaller than 1/8 inch in thickness goes into a grow-on tray in the mini greenhouse to plant out once they grow bigger. I seldom have much loss using these.
I've talked with other gardeners who say they have lots of loss using these seedlings. I'm not entirely sure what they are doing wrong. My method:
...remove them from the shipping box immediately.
...store them in a cool, dry, airy spot until I plant them. That means on the floor of my north facing lanai.
...plant them as soon as possible, usually within 2-3 days.
...plant the larger ones directly into the gardens. Plant the smaller ones, less than 1/8" diameter, into growing trays in the mini greenhouses.
...make a point to keep the soil lightly moist but not overly wet for the next week or so.
...apply only a very light layer of mulch until the little onion plants have grown some. Don't bury them in mulch.