I have not sprouted my own onions but have purchased both onion and leek transplants. They came with numerous small bulb seedlings growing together in a cell pack. I bought one small container that must have had 100 leeks growing together in it - for $4! What a deal :-)
When I am transplanting them I gently shake the soil off, then swish them in water to remove the rest. I tease them apart and you can trim down the roots a bit and the tops too with scissors. This is supposed to help make a larger bulb.
Then I planted them as usual. For leeks I make a larger hole, then add a smaller hole where the seedling will go. This is supposed to help with growing more white bulb part and less green stalk.