Heat oven to 200C/180C fan/ gas 6. Rub the peppers with a little oil and roast for 30 mins, turning halfway through, until soft and slightly charred. Place in a bowl, cover with cling film and set aside.
Meanwhile, heat a large griddle pan (or two, if you have them, for speed) until hot. Drizzle the courgettes and aubergine with oil, then griddle for a few mins each side until char lines appear. Peel the peppers and discard the seeds. Tear the peppers into thick strips.
Spread the houmous over a serving plate. Mix together the lemon juice and garlic. Toss with the vegetables and half the sumac, then arrange over the houmous. Top with the rocket leaves and sprinkle over the remaining sumac. Serve with ciabatta.