Let the pulled onions dry in the sun until the roots are brittle, then wipe off any excess soil and remove the tops unless you're planning on braiding them to store. Store in a mesh sack hung from the ceiling of your cellar for the best air circulation. Try this recipe for Onion Soup . Garden clean up - Compost leftover tops and debris. Next year, plant your onions in another part of the garden.
Crop rotation discourages diseases.