Why not let them sprout, and plant. Then next year you will have your own seed.
Yes, I’ve re-planted six of last years JLK to go to seed this year for sowing next year....... hopefully! If I’m successful I’ll be back here beginning of March next year and should have plenty to spare.
Would you sow them with bottom heat or just in a cold greenhouse?
Either really wherever you have room, if we have the same weather as last spring they could be sown outside in a couple of weeks or so. The problem I find with onions is that they don’t like being messed about with too much. So the younger they are when they are transplanted the better. Unlike leeks where you can knock ten bells out of them and they just come back for more.