Blue Cheese, Peanut Butter and Celery Leaf Pesto

Celery leaves are the big, flat, blousy leaves that adorn the sides of fresh celery crowns. Some supermarkets cut them off, which makes me wonder where on earth they end up, but most of the cheaper packages leave them intact. I use them as a salad leaf in their own right, to flavour cold jugs of water in summer, whizzed up with oil and salt and vinegar to make an all-purpose dressing, and sometimes in this rather lah-di-dah pesto. You can make it more of a dip by thinning it with natural yo