Red Baron's are well known for bolting early if they do not get optimal growing conditions. I put my red onions in either in October, to get established before winter hits, or in March when hopefully the ground has warmed up a bit!
I don't have any experience in growing onions under fleece, I guess you could try covering the ground with polythene to warm it first for a couple of weeks and then plant and fleece. They should enjoy the manure from last years potatoes, I also use chicken manure pellets in Spring to give them a boost. Or you could plant under cover, in a cold frame or polytunnel. Personally, I'm waiting until March to put my next batch in! ;)