If the garden bed isn't fertile and doesn't have good structure then the plants will struggle to survive. Onions take 24 to 32 weeks to reach maturity (depending on variety etc) and need enough nutrients to sustain them during the long growing period
We put grass-fed animal manures (we never use Factory Farmed manures as they are toxic in my opinion), mulched up leaves, Blood& Bone, Dynamic Lifter plus a bit of straw from the hen house straight onto the beds after each harvest and then let the beds rest during winter . This allows the organic material to decompose. The blood and bone is essential because it replaces the nitrogen that is lost during decomposition. Allowing the beds to rest after each growing season is essential to maintaining the health, soil structure and fertility of our soil.
If you don’t have access to grass-fed animal manures and don’t want to rest the beds then you can
1) buy a bag of cow poo from the shop and put that around the onions. It will add a small amount of fertilizer but will help improve the structure of the soil which in turn helps with moisture retention & micro development etc .
2) Buy some Blood & Bone which is a powdered animal product from abattoir. It replaces nitrogen that can be lost during decomposition of organic material such as straw etc
3) Buy Dynamic Lifter which is dried chicken poop ..it probably comes from "Factory Farms" so may contain residues but I don't really know. I use it even though I have poop from my own chickens
What you are after is constant growth ... not too fast and not too slow
4) you could buy a "Complete slow release Fertiliser" which will contain all the necessary chemical elements essential for plant growth and will release over time ("Complete Fertiliser doesn't lock up nutrients either) . There is nothing wrong with using a complete fertilizer except for the fact that it doesn't add any organic matter to help with soil structure. ... I don't personally use it on my vegetable garden because the area is too big and would cost me a fortune but on small gardens, raised beds or pots its fine.
5) I never use "fish emulsions" or "liquid soil conditioners" because they can cause rapid sappy growth which can attract Aphids (that's my opinion)
6) you could do green manures (grow legumes or rye etc and dig them in blah blah )... I don't do green manures because I don't want to, doesn't suit what I'm doing ... I much prefer to allow the beds to rest after each harvest