Chicken and barley casserole

Ingredients

• 1 teaspoon vegetable oil
• 2 carrots, peeled and finely chopped
• 2 leeks, finely sliced
• 2 celery sticks, roughly chopped
• 1 clove garlic, finely diced
• 2 sprigs fresh rosemary, finely chopped
• 1 bay leaf
• 200g pearl barley
• 500ml reduced-salt chicken stock
• 500ml boiling water
• 3 chicken breasts, roughly chopped
• Freshly ground black pepper