Yes. Cauliflower and Onion have a documented companion-planting benefit. Virginia Cooperative Extension lists onion as a companion for the cabbage-family crop cauliflower.
This is a practical yes, not a magic guarantee. Keep the normal spacing for Cauliflower and Onion, watch airflow and watering, and treat the companion note as one useful planning signal alongside sunlight, soil, and harvest timing.
Source basis: Virginia Cooperative Extension companion planting chart. The page does not add extra pest-control or yield claims beyond the sourced fact row.
Spacing tip: Use the normal spacing for the larger crop, then tuck the smaller crop where it will still get light and water.