Cheese & onion pasties

Name: Cheese & onion pasties
Prep time: 30 min
Cook time: 35 min
Total time: 1 h 5 min
Cuisine: European, British
Ingredients: 1 tbsp vegetable oil; 2 onions sliced; 4 thyme sprigs leaves picked; 150g mature cheddar (or a mixture of strong hard cheeses), grated; 1 tbsp wholegrain mustard; 4 spring onions sliced; 250g plain flour plus extra for dusting; 125g cold butter; large pinch of cayenne pepper; ½ tsp English mustard powder; 1 large egg beaten