Dieter, interesting question. Onions are biennial, so flowering in their second season (or when they think it’s their second season!) so sowing after Christmas for a summer harvest should not result in any bolting. I think OW onions are heat treated which kills the dormant embryo in the baby bulb resulting in few actually bolting, but I could be missing some other knowledge here.
Rhys, top grower on here, has saved some of his own seed for exhibition growing and is running some trials this year, sowing in January I think – check it out 🙂