I'm not an expert but I have had great success growing onions and leeks in large pots like in the pic. Those are my leeks growing! I had horrible damping off problems too when I used small cell trays or even small containers. Leeks and onions have long roots and they thrive in these big containers.
Here is what I do: not that you asked lol
Fill the big pot with good potting mix, leaving about 2 inches headspace.
Sprinkle the seeds generously all over the soil.
Sprinkle more soil on the seeds so they are about 1/4 inch under the soil.
Water with a sprayer.
Place a saran wrap over the pot and place it on your seedling mat and under a light. They will emerge in less than a week! Once you see the first few seedlings pop up, remove the saran wrap and place the pot under grow lights.
I've not had to water at all with the saran wrap method until after the seeds germinate. If you notice that the soil is too wet, remove the wrap and let the soil breathe for a day or so.
And like
@AMKuska said, leeks and onions are very hungry crops so I feed them every 2 weeks. I use water soluble fertilizer and bottom water.
Pic on the right is onions.