About ten days ago I put four varieties of seed in petri dishes - paper towel lined, dampened, then turned sideways on to drain off excess water. This weekend I am pricking out the seedlings into 8" pots of compost. The target is transplantation in mid to late April.
The pots of compost/sprouted seeds will be kept in a large frame. If really cold weather threatens I'll bring them indoors.
The purpose of this labour intensive method is to initiate lots of "layer formation" in the seedlings before the daylight/nighttime ratio gets too high - at which point the seedlings stop producing new layers and concentrate on fleshing up. The more layers in an onion the bigger the onion, others things being equal. (Don't try to achieve big store onions by using lots of nitrogen; they won't keep - too watery.)
There is still time to use this method, if you get moving.