Pork chops with celeriac mash and apple and ale gravy

INGREDIENTS

• 1 large celeriac
• 150g/5oz butter
• salt and freshly ground black pepper
• 1 large onion
• 2 tbsp extra virgin olive oil
• 3 Cox's apples
• 425ml/¾ pint London Pride ale
• 4 pork chops
• 2 tsp vegetable oil
• ½ tsp Dijon mustard
• 1 tbsp white wine vinegar
• 3 tbsp extra virgin olive oil
• 250g/9oz rocket