Make the pesto. Put the walnuts, garlic and lemon zest in a food processor and whizz until finely chopped.
Add the parsley, then drizzle the oil down the feed tube and whizz to combine everything together. Season to taste.
Preheat the grill to high. Tip the onions into a grill pan, sprinkle with 1 tbsp oil, then toss to coat. Arrange them in a single layer and grill for 6-8 minutes until lightly charred.
Meanwhile, cook the pasta according to the pack instructions until just tender – it won’t swell as much as wheat pasta, so taste a piece to check that it’s just tender.
Drain the pasta in a colander. Tip the pesto, onions and peppers into the pasta pan and put back on the heat. Stir for a few moments, then add the pasta and then the cheese. Season and serve straight away, scattered with the cheese shavings.