The first 2-3 years, we tried onion sets and had disappointing results -some grew, but never very well. Now I know that onions are day light dependent or neutral. But sets never seem to be labeled with this info. So, on a whim, I tried seeds last year. A long day and a neutral day. Bc I was ill for awhile last spring, things were started and planted a bit late. So, unsurprisingly the neutral day onions did a bit better than the long day. But both outperformed the sets I had used in all previous years. Onion seeds are supposedly only viable for a year, but probably the viability just drops off quite a bit after a year, and some are still good depending on storage.
I’m using the pointy-type cells -72 cells per tray. I plant 1-2 seeds per cell. They will grow and it’s my understanding I can plant before last frost date, but I need to check that. If you grow seeds and the onions are tall, cut them back to 5” tall before transplant. If you have 2 viable plants, tease them apart. Because an onion is actually a modified leaf, and it is sunlight that causes the bulb to grow , they are not as affected by transplanting or being in a cell a little too long. Not like actual root veg which really need to be direct planted or direct seeded. Even though mine were planted quite a bit after last frost, I got some nice sized bulbs for use.
When you plant, you can start a lot of seeds, plant them close and then thin to get some small onions/green onions for fresh use. Or plant a distance apart and let them grow.
From experience last year, the ones that received more shade (due to being in closer proximity to another plant) grew less or just shriveled/no growth/death. So-put in full sun.