Apple, cheese & potato pie

Name: Apple, cheese & potato pie
Prep time: 25 min
Cook time: 1 h 15 min
Total time: 1 h 40 min
Cuisine: European, British
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