I would guess that you need to turn over the soil to a fair depth as the soil will have become compacted over time. The recommendation is to double dig it
and to incorporate manure at the time.
This leads me to wonder if you intend to edge the area with anything or if you intend to develop it as a raised bed. Edgeing it would separate it from the remainder of the grass. Depends on where the new veg area is positioned.
Winter is basically a time to prepare the bed for the next year and at the chance of being told several alternatives I count it as digging, incorporating manure/fertiliser and then leaving it to rest, while planning the next round of planting. Just done mine about 10 hours ago.
Now to plan.