Mix olive oil, rosemary, garlic, lemon juice and zest in a large bowl. Add pork, turn to coat and season well. If you have time marinate in the fridge for 30 mins.
Heat a large non-stick frying pan. Lift the pork out of the marinade, shaking off any excess and reserving the marinade for later. Cook the pork in the pan for 3-4 mins each side or until cooked through. Rest on a plate while you make the salad.
Pour the reserved marinade into the pan with the onion. Cook for 1 min over a high heat before adding the vinegar, plus 3 tbsp water. Bubble down for 1 min, until the onion has softened a little and the dressing thickened slightly. Stir through chickpeas, some salt and pepper and any of the resting juices from the pork. Put salad leaves into a bowl, tip in the pan contents and gently toss, before eating immediately with the pork.