My onion bed this year had sweetcorn on last year. It had some compost last year but was dreadful- a mix of shale, clay, and sand used as a base for a pigeon shed. Pigeons now gone, so to improve it I gave it a 6 inch layer of very well rotted horse manure, with a dressing of 1kg(shovel full) per square yard of rock dust for micronutrients, 100g (cupful)per square yard of calcified seaweed, 100g per sq yard of fish blood and bone. I then mixed it up well, forking it over twice. I then planted onion plants I grew from seed into it, and watered them in.
For results see verduns onion thread