Welsh onion cake with caerphilly - Good Food Middle East

Ingredients

• 200g butter, plus extra for the tin
• drizzle of oil
• 3 large onions, thinly sliced
• 1 leek, sliced
• 1kg floury potatoes, such as Maris Piper or Désirée, peeled and thinly sliced (use a mandoline or the fine slicing blade of a food processor)
• 150g Caerphilly cheese, crumbled (or another sharp, crumbly cheese)