As I found out about growing onions from seed, it is important to start early and then patiently keep re-potting the seedlings. It is supposed to strengthen the root growth and that will help as I eventually plant them out later in the year in the prepared open ground.
The seeds were sown in some standard potting compost and kept on a windowsill in my kitchen. they soon came through and I decided to single them out into small modules. My greenhouse in unheated but lined with bubble plastic so I’m hoping they’ll be ok there. I’ve covered them with a clear lid and watered sufficiently so all that remains now is to wait and perhaps be ready to come to their rescue if the weather turns too nasty even for my greenhouse.