A rich beef stew that gets better the longer it’s kept. The pickled walnuts add a tangy touch. Make up to three days aheadIngredients:3.0 tbsp olive oilFOR THE STEW3.0 tbsp chopped fresh flatleaf parsley , plus extra to serve85.0g butter3 large Spanish onions , finely chopped2 garlic cloves , peeled and crushed1½ kg stewing beef…