I`ve always started my onion sets,and shallots under glass,and in trays of compost.
Only yesterday i started to plant my onion bed out,- with approx` 200 sets going in,and all boasting a growth of green stem of approx`5".
The shallots are still in trays of compost ready to go in,and are about 4"high in green stem.
I`ve never had any problems over the years "kick starting"my sets.
Regards,
Mark.