I'm not quite sure what you're doing. Did you peel up all the sod, & turn it over in place? If so, I would mix the soil & manure and put it on top. A friend of mine puts the soil down, then the manure on top, instead. She says the nutrients will wash down, anyway. I think it tends to dry the manure out too much, but that's just my opinion.

You can plant onion seeds directly into the ground as long as you can keep the soil uniformly moist so they don't dry out when they're partly sprouted. You might try covering the onion bed with some burlap, and spraying it several times a day to keep the soil moist underneath. As soon as you see sprouts, remove it.

Sue