Fill a kettle and boil it. Trim the leek. Slice a cross into the top of it so you can open it out. Rinse out any grit. Slice the leek in to 1 cm rounds. Thinly slice the mushrooms.
Warm a large frying pan. Add 1 tbsp olive oil. Add the leeks and mushrooms. Stir and fry for 5 mins till both vegetables are soft and glossy.Take off the heat.
While the veg cook, pop the gnocchi in a pan. Pour in enough hot water from the kettle to cover them. Turn the heat up high.
Simmer the gnocchi for about 3 mins till the gnocchi float to the surface. Drain straight away - if you cook them for too long they'll end up heavy and soggy. Add them to the veg in the frying pan.
Chop the Perl Las into small cubes. Add it to the gnocchi. Set a handful of rocket aside for later. Add the rest of the rocket to the gnocchi and gently toss, so it wilts and the cheese starts to melt. Season.
Finely grate the zest from the lemon. Add it to the pan. Stir well. Cut the lemon into wedges.
Put the pan under the grill for 3-4 mins till golden and bubbling. Top with the reserved rocket. Serve straight away with the lemon wedges for squeezing.