Whisk together the mustard, vinegar and
oil in a small bowl until well combined. Stir
in the double cream. Peel the celeriac and
cut into really fine strips using a mandolin
or very sharp knife. Then cut into matchsticks
about 5cm long. Mix into the dressing along
with the parsley.
Cut each prosciutto slice in half down
the middle, then cut each piece lengthways
so that you have 4 small strips. Place 1 tsp
of the celeriac mixture onto each strip, then
roll them up tightly. Secure each strip with
a toothpick, place on a plate and leave in the
fridge for at least 20 mins to firm up. Can
be made a few hours ahead and kept in
the fridge until ready to serve.