Line a grill pan with foil and lightly brush
with oil to stop the salmon sticking. Add the
salmon and spread each fillet with 1 tsp
mustard. Curl up the edges of the foil to
catch any juices, then squeeze over a little
lemon juice. Heat the grill to medium.
Boil the celeriac and potato for 15 mins.
Add the peas, then cook for a further 5 mins
until the vegetables are tender. Meanwhile,
grill the salmon for 12-15 mins.
Drain the vegetables, reserving a cup of the
cooking water, then mash roughly so that
the peas start to break up. Add a little of the
cooking water to loosen the mix, if you like.
Season to taste, then divide between two
warm plates and top with the salmon. Pour
over any pan juices, then serve.