Celeriac gratin

INGREDIENTS

• 1 kg potatoes, peeled and sliced into 1cm slices
• 1 large celeriac, peeled and sliced into 1cm slices
• 1 onion, peeled and finely sliced
• sea salt
• freshly ground black pepper
• 2 cloves garlic, peeled and finely chopped
• 75 g Cheddar cheese, grated
• 600 ml double cream
• 1 small bunch fresh flat-leaf parsley, leaves picked, stalks roughly chopped