Sow between onion rows
Sow a crop in the space between your onion rows. Onions will be harvested in a few weeks and they create very little soil disturbance when pulled from the ground, making them ideal for intercropping like this.
Either grow small upright plants like little gem lettuce, which are cut rather than pulled, or sow sweetcorn or beetroot direct, which will only start to get big once the onions have gone.