Rinse the maftoul, then tip it into a small saucepan and cover with 200ml water. Cover, bring to a boil on a medium-high heat, turn down to a simmer and cook for 15 minutes, until al dente (if you’re using a different type of couscous, it may well cook much faster than maftoul, so follow the packet instructions). Take off the heat and leave to cool.
Roughly chop the celery leaves (if need be, make up the weight with flat-leaf parsley leaves), finely shred the leek tops and roughly chop the roast peppers.
Stir all these into the couscous, then dress with the olive oil and lemon juice.
Add the dried mint and sumac, if using, season to taste with sea salt, toss again and serve.
