I used to be a fan of overwintering onions until I figured they were probably hosting the mildew which almost wiped out my spring planted and sown onions in 2005-8. Since then I stopped sowing onions in the last week of August (usually Senshyu in fact) and just sowed them in February in the greenhouse, or planted sets late March to early April. This year’s spring sown and planted onions are mostly good apart from one variety (Marco) which appears susceptible to mildew.
To overwinter, do not sow before August 23rd or they risk bolting next spring. Before 31st gives them time to make plants for setting out in late September. Spring onions use the same dates, White Lisbon is good. Onions do not germinate well in temperatures consistently over 20C, so keep the trays out of sunlight until you see green shoots.