Is it hard to grow leeks and onions in pots for transplanting into the garden?

Get one of the 1200 set ups on flat watertight, the other that has holes and another with 12 cells, doesn't make any difference in the number of holes in the cells, it's just for rigidity and get a dome , you can buy the whole set , but since I buy them by the hundreds, I my set will cost me about 2. dollars, yours will probably be more than double that, but then get some potting mix with fertilizer in it (I get Miracle Grow with extra water retention) put the flat with the hole in the watertight flat, then the 12 cell in cutting one out to water from the bottom, they are almost cut out anyway. fill the flat with potting soil to the level of the cells then put in the onion seeds however close you want them, sprinkle potting soil and gently tamper. Take out that one loose cell and put nice amount of water with some dish soap with bleach in it. Then put the cell back in and cover with the dome. Then I place them under fluorescent shop light with a warm and a cool white bulb. I raise them by the thousands to sell and all the stuff you raise too. Just couldn't handle all the one I raise in cake pans but its a good idea. Just use what you can from my system and ignore the rest:) Actually wanted to explain I didn't see your cake pans and such till I'd already written the answer, I leap, and the look