You have two options, Geezer:

Option 1. - If you can't save an Ailsa to plant next year because it's a type that doesn't keep til planting time in the spring, then plant it indoors whenever it's ready to start a new life [you see a green line down the centre of the cut onion, or it sprouts]. You don't even need soil: I just set mine on top of a shallow pan of pebbles with water just below the top row of pebbles. The roots reach down into the water, just like daffodil or narcissus bulbs. At outdoor planting time, pull the onion out of the pot, leave any pebbles clinging to the roots, and plant as usual.

Option 2. - If you buy flats of Ailsa to transplant into your garden, try re-transplanting a few after they have shown some growth, say a week or two later. Cut back the greens, leaving only an inch, and trim back the roots, leaving one inch, then replant. This usually shocks the plant to bolt.

I grow potato onions [multipliers]. Most I plant in the fall and cover with mulch and a screen to keep the squirrels from digging there to bury their peanuts - but I always save a few backup onions to plant in the spring.

I frequently find that some spring planted ones have been pulled out of the ground as soon as one green shoot pops up [by crows, I suspect, hoping it's corn] and I just replant these as I find them. Inevitably, these "already rooted but disturbed and re-planted" onions are the ones that produce seed - and potato onions are reputed to very rarely go to seed. It is from this experience that I suggest you try Option 2.
Sylvia

Sylvia---Thanks very much for answering. I will try both options to make sure I get some seeds. Geezer

You may 'fool' them by putting some in the refridgerator for a few weeks (the plants, that is). Not sure on the stage you would put them in the fridge (I would let them grown a bit in the garden), or exactly how long you would do that, but it should work in theory.

But, Ailsa Craig will readily overwinter in the garden in your zone especially if mulched. I did it in Northern Wisconsin. So you could collect seeds next year. They produces many seeds! In fact a 'seed patch' can be established - as they will actually be perrenial in the garden if cared for a bit. - scrape off the old layers and replant them in the spring when they start to come up. They will never produce large onions when overwintered, but will produce seeds every year!

They grow huge! I plant seeds inside in December and out in the garden as soon as possible (usually late March or April).