In a large pan of boiling salted
water, cook the parsnips and
swede, covered, for about 20
minutes until tender. Drain well,
then mash them together using
a masher or food processor until
reasonably smooth but still with
a bit of texture. Stir in the soured
cream, horseradish and thyme and
season with salt and pepper.
Spoon into a buttered shallow
ovenproof dish and put to one side.
Make the topping. Melt the butter
in a frying pan and cook the onion
for 5-6 minutes, until gorgeously
golden. Mix in the breadcrumbs
and stir to brown and crisp a little.
Season with salt and pepper and
add the thyme. Take the pan off the
heat. Spoon the mixture casually
over the top of the mash. Scatter
over the parmesan. (Can be made
ahead to this point and kept covered
in the fridge for up to a day. Or can
be frozen for up to a month.)
Bake with the turkey (190C/gas 5/
fan 170C) for 35-40 minutes if
doing from cold, 25-30 minutes
if not, or 11?2-13?4 hours from frozen
(put foil on top, and remove it for the
last 10 minutes) – until golden and
crisp on top. Serve scattered with a
few more thyme sprigs and leaves.