I have no
experience growing onions. But I planted potatoes and some flowers using Eric's fertile hole suggestions (essentially a hole filled with
compost and the onion set). You don't need as much bagged stuff to do that, so you can spring for a more expensive compost. Add some shredded leaves, maybe, or leaf mold if you have some of that.
I had moderate success with the potatoes. Some of them rotted, so I conclude that (due to the drought) I was watering too much!