What you want now is a cover crop that will grow some in cool weather, stay in place over the winter and grow a bit more next spring, something that will help hold your soil in place during the winter. That means knowing a bit about which cover crops will do that. Oats are seeded in the spring to grow during the summer, they need warm weather and are killed by frost. Buckwheat is the same. Wheat maybe, depending on which wheat seed you get. Many of the wheats have been genetically modified and may not be something you would want, and that seed is fairly expensive which is why that is seldom listed on Cover Crop lists. Most clovers prefer to be spring seeded so the have time to get established before winter sets in. Fava beans, and Austrian Peas, like to grow in cooler weather, but are killed by really cold weather so only dead plants would be left on the soil during the winter. Winter, Field, Cereal Rye, "Secale cereale", is one grain that germinates well in cooler soils, grows 4 to 6 inches tall, and then stops growing until the soil warms up. This helps hold your soil in place if snow cover is insufficient to keep surface soil from blowing, eroding, away. This is one of the few grains, or cover crops, that will stay alive, along with Winter Wheat, and will actually provide cover, the reason you plant a cover crop, for your soil.

Depending on where you are. Timing is very important. Type of onion is very important. Fertility of soil soil is important. Start with a well prepared soil. It needs to be loose and friable. Onions are heavy feeders especially of nitrogen. They are essentially a leaf vegetable. The onion bulb is essentially the base of the leaf. If you you are in the north, use long day plants like Spanish types which grow large. In a border states use a day neutral plant like Candy. In the deep south use short day plants of the Granex or Grano type. These need to be winter grown, so transplant in late fall (November- December) for best results. The other types other types for mid and northern states as soon as the ground can be worked. That means after the spring thaw and ground has dried out a bit. Depending on your area that can be late winter-early spring

Thanks so much, Planatus. Plan to order a few of your suggestions. Last fall, I also broke up several clumps of chives and had phenomenal reproduction and enjoyment of these ...and still do. I had never had such huge and tender chives before from these plants that I have had for decades. Seysonn, the onion sets' growth in our HT last fall did not behave as those in your experience. It seemed as though the HT going into fall/winter was absolutely perfect. Only 4-5 bulbs out of hundreds had a flower stalk. Even those were tasty in fritattas. I let them bloom to see what would result. One had black seeds which I will test for germination in a week or so. Some folks say that growing zones make a huge difference in alliums in High Tunnels, while others say it is more of a light availability that determines the production of the fall-winter crop. Some suggest recording light availability by automatic methods, but I am not so advanced and am still learning how to assess soil moisture levels by inexpensive/simple tests. Our central KY site had abundant fall-winter-spring sunlight this first trial. Have sent emails to a few HT gurus in hopes of learning more about allium possibilities. One person in MN-zone 3-said that by using agribon and a plastic sheet over wire hoops, he managed green onions thru the winter-alas, variety not specified. He also did not say if onions were grown thru the 1 ml black plastic. Have not yet located a USA High Tunnel(unheated) index that covers varietal issues for all zones for all vegetables. Not even all states have a listing yet, from my research. Our NRCS HT has only a single 6ml cover, so it's results may be varied compared to the double layered HT covers used by many commercial growers. This fall, plan to do a short row with garlics, elephant garlic and one with yellow potato onions, as well as the winter onions planatus mentioned. Will do a similar test planting outdoors in a raised bed. We eat alliums at least twice a day-so can never have too many!

Do you know what variety of onions they are? I assume you are somewhere in the south where short day onions can be grown thru the winter. I believe mid/late April is usually when the growers in Georgia harvest the short day Vidalia onions but then I purchased some at the grocer this past weekend so it maybe earlier. If these were not a short day type then that may be why they had not bulbed up yet. They do look like they might have just started to bulb before you pulled them. I don't think you will have any luck with putting them back in the ground but it wouldn't hurt to try I guess. When onions bulb, about 2/3 of the bulb is visible above ground as long as they were planted at the correct 1" depth so it is easy to determine how far along they are. Drmbear gave you good information on how to tell when they are ready to harvest. Tough lesson to learn but gardening is a never ending lesson!