Fish, Blood and Bone.

I hope that I will be putting my shallot and onion sets in within the next few weeks but my soil is starved of nutrients. I realise that there is nothing better than compost but I will not have anything ready until next year. Would it be okay to give a dressing of Fish, Blood and bone a week or two before to improve things?

Yes - that's good stuff. Also, a dressing of calcified seaweed if you can get it to make up for any lack of trace elements.

Just one point about compost - as it's organic matter, it's great for the soil structure but isn't that good on the nutrient front.  As you identified, you need a fertiliser of some sort to enhance nutrient levels.