Answer: There are several soil insects which can feed on the roots of garden plants, including your onions. Two likely culprits are wireworms and grubs. Wireworms are about an inch long and reddish-brown in color. Grubs (the larvae of "June bugs") are "C" shaped, white and usually about 1" across when curled up.
When you dig up a plant, look through the soil around the damaged portion for the culprits. Depending on what you find, control measures can be suggested to fit the pest. You may submit another question via the web site with a description of the pests if you like.
The problem may go away on its own, as such damage to onions in Texas is not that common. Next year's crop may go unharmed. Insects have a way of tending to have occasional outbreaks and then go away for a time. Therefore, recommending an insecticidal treatment of the soil every time you grow onions may be unnecessary.
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