The perfect potato gratin

INGREDIENTS

• 1 small knob butter
• 200 ml semi-skimmed milk
• 300 ml double cream
• 2 bay leaves
• 2 cloves garlic, peeled and finely sliced
• sea salt
• freshly ground black pepper
• 2.5 kg potatoes, peeled and thinly sliced
• 1 handful fresh thyme
• 1 small handful Parmesan cheese, freshly grated
• olive oil
• 6 rashers higher-welfare streaky bacon, chopped
• 1 handful vac-packed chestnuts, peeled and crumbled