Around one edge of my garden, I have a perennial allium bed to provide plants each year for the garden. Alliums are the onion-family vegetables - onions, shallots, garlic, leeks, and some raised just for their flowers. I just wrote about saving the best of my garlic bulbs to plant each fall for the next year's crop. I'll also do that with the shallots now ready to harvest. Big bulb onions are about the only plants I don't propagate myself. My local nursery sells onion plants by the bunch in early spring. I buy a bunch or two to set out so I'll have lots of onions when I'm ready to can batches of salsa and tomato sauce in late summer.
These are walking onions, also called top-set or Egyptian onions. The plant makes a little bunch of onion bulblets at the top of a stalk, sometimes reaching two or three tiers high. If left alone, the stalk falls over and the bulblets grow where they land - the plants thus "walk" around the garden over time. The plants are extremely hardy and the first onions to come up in the very early spring. I use them then as green onions, but later in the summer they can get tough. I plant a couple of the top-sets each fall to replace the plants I've used.
I also grow bunching onions - multiplying clumps of green onions that never form a bulb shape - that come up later in another part of the bed. These are the scallions I use throughout the summer, making sure to replant one or two, for next year's supply, when I pull up a clump. Leeks are the last of the onion season. I harvest them fresh all through the winter, digging them out of the frozen ground. The last section of my perennial allium bed is for my leek supply. Once you grow one leek, you'll never have to start seeds or buy plants ever again.
Years ago, I transplanted 6-8 leeks into the supply bed. The following year, they sent up a big flower stalk, but also a clump of little leeks came up from the root. Early each spring, I'll pull up a clump, break it apart, and set the little plants out in my regular garden bed for harvest later in the year. I'll put one of the last holdouts from the year before in the propagation bed to replace that clump, leaving the rest to continue growing into even bigger clumps. This method even works with the cut-off root end of a supermarket leek. Don't throw it out - bury it out in some place where it can grow undisturbed. It will come back as a clump of little leek plants, ready to plant, the following year.