Apple, cheese & potato pie - Good Food Middle East

Ingredients

• 30g salted butter
• 1 tbsp vegetable oil
• 2 large onions, halved and finely sliced
• ½ bunch of thyme, leaves picked
• 30g plain flour
• 500ml vegetable stock
• 1 tbsp wholegrain mustard
• 1 tbsp white wine vinegar
• 450g potatoes (we used Maris Piper), cut into 2-3cm chunks
• 3 apples, peeled, cored and chopped into 1-2cm chunks
• 150g mature cheddar, grated
• 300g plain flour, plus extra for dusting
• 70g cheddar, grated
• 150g cold butter, cut into cubes
• 1 egg, beaten