Put the peppers, skin side up, on a foil-lined grill rack. Brush them with a little of the oil and cook under a preheated grill for 10—12 minutes or until they are softened and the skins charred. Put the peppers in a plastic bag, fold over the top to seal and leave to cool.
Meanwhile, make the dressing by mixing the remaining oil with the vinegar, tomato paste and chopped capers. Season to taste with salt and pepper.
Rinse and drain the beans or chickpeas. Stir them into the dressing with the onion and celery.
Peel the skins off the peppers and cut the flesh into strips. Add to the beans and gently toss together. Crumble the feta cheese over the top and serve the salad on a bed of lettuce leaves.