Boulangère potatoes

INGREDIENTS

• 1.5 kg Desirée potatoes, peeled and thinly sliced
• 2 onions, peeled, halved and thinly sliced
• a few sprigs of fresh sage, leaves picked and finely sliced, some leaves left whole
• sea salt
• freshly ground black pepper
• 400 ml organic vegetable stock
• 1 large handful of Parmesan cheese, freshly grated
• 1 knob butter