I planted shallots last fall, sometime in October. Used some bone meal with them, and mulched them with straw once they took off growing. Just finished harvesting.
Last week I planted a bed with red and white multiplying/walking onions that I got at the Roundup. I didn't find anything that said to wait until fall to plant; a few sources said to divide in spring... I'm eager to read if there's anything special I should (should have?) done to them.
I have tree onions (multiplying onions/walking onions). I also didn't know to plant them in autumn but planted them in spring 2001. They didn't do much the first year but are now reproducing like mad and I am letting them do so, guiding them to fill a bed and then I might think about thinning them. The lack of growth the first year indicates I maybe should have left it until autumn.
I find autumn planted normal onions (red or white) don't store well so plant those in April.
I always plant my shallots early spring (March or April, depending on the snow and ice), rather than autumn; they wait until May to start really growing and then grow rapidly - they're ready for harvesting now.
Fertiliser - autumn or spring, just home-produced compost and watering if there is a shortage of rain. My shallots this year grew with coriander which certainly hasn't done them any harm.
The only allium I plant in autumn is garlic as I find it's the only "onion" that benefits from autumn sowing. It usually follows lettuce or brassica but this year is also following tomatoes.
My compost is usually 1.5 to 2 years old and composed of kitchen and garden waste with the addition of comfrey, nettles and some cow muck/straw mixture. The onion family seems to like it.
Potato onions are the bulbing type that make smaller but good size bulb onions from an individual set. This is the 3rd year growing them this way it's working well. I mix in aged compost, after thoroughly digging the bed. I sprinkle a handful of organic fertilizer and rake in. The sets are planted 2" deep or so and mulched heavily with dried lawn grass. They will be ready for harvest by the end of the month.
I used to plant regular red and yellow onion seedlings in fall. I learned after a few years that too often the winter frosts trigger the start of seed production in the onions in spring. I now plant onion seedlings in late Dec or Jan.
The Walking onions have been in their permanent bed for 4 years now. they have been thinned and fertilized in late winter with compost only.
I too have been raising the Walking onions for a long time. I plant the bulblet anytime of the year and they take root and do fine. I don't do any fertilizing other than what I give everything else in the raised beds and flower beds.
I also plant the Egyptian onion (amazing how many different names it has!) and garlic in the fall. I also plant them in mid-summer, early winter, late winter, mid-spring, basically, anytime in the year!
I have found that with me, it makes no difference when/how I plant them, but we have a fairly mild climate, so I don't have to do anything special for protection. I plant chive seed in the fall, and also regular onion seed in the fall.
My veggie garden is mostly compost, about 10" deep over clay. It gets mulched with 4" of chopped leaves every fall, then a compost mulch 3" deep in the spring.
I don't add any additional fertilizer to the soil, but I do sprinkle with compost tea once a week during the growing season.