onions are easy to grow and good to store. and in addition, they are great supporters to fight pests of neighboring vegetables, such as strawberries, carrots and lettuces.
of course you can grow onions from seeds but as it is very tedious, i never tried it. it will take two seasons until you get an onion out of a seed. that´s why we buy the small bulbs to plant in spring.
plant them in a row next to a row of carrots for example. they will need 10 cm distance to each other in the row and 20 cm distance to the next row or plant them in the space between your lettuces or strawberries. make a small hole, just big enough to put the bulb in it so that the small tip will still surface.
in this manner you can grow every type of onion, yellow or red once, white onion, pearl onion or even shallots. just be aware that shallots will need more space, for about 25-30 cm between the rows, because they will build nests of new shallots, while the onion bulb simply will grow.
they are ready to harvest when the greens start to wilt. pull them out of the ground and put them on your vegetable patch to dry a bit. you should harvest onions on sunny days only.
for good stocking conditions you should make brunches of onions. fix every brunch with some string on the greens and hang them for some weeks on a dry and airy space.
after some weeks they are ready to be stored in your cellar. if the conditions are good (dark, dry and cold) they will remain firm until the end of winter.