Here are the summary of some of the things that you must get right to achieve high yield.
- Procure high yielding variety seeds (hybrids)
Cultural factors
- prepare land early enough, to enable timely planting, weed management for the crops to benefit optimally on available moisture and nutrients
Correct spacing
Soil factors
-ensure soil PH matches the required range
- right type of soil(sandy loam,deep,fertile and well aerated)
Another thing to consider is irrigation possibility to supplement irregular/insufficient rainfall if it turns out to be so.
Nutrients supply through fertilization this you can follow the report from your soil test result to enable you be privy to which elements are in insufficient state and how much is needed as far as onion production is concern.
Please read more on the links provided below on nursery and diseases and pest management and you will be good to go.
https://www.mkulima.co/threads/managing a onions diseases