Sage, leek & onion balls

INGREDIENTS

• 4 tbsp vegetable or sunflower oil
• 2 onions , finely chopped
• 1 leek , finely sliced into rings
• 2 celery sticks, finely chopped
• 450g pack good-quality pork sausages , meat squeezed out
• 20g pack sage , leaves finely chopped
• 140g white breadcrumbs
• 1 large egg
• 8 smoked dry-cured streaky bacon rashers, halved and stretched out a bit
• sage sprig, to garnish (optional)