Cheesy leek & potato pie

Name: Cheesy leek & potato pie
Prep time: 10 min
Cook time: 1 h
Total time: 1 h 10 min
Cuisine: British
Ingredients: 3 leeks, cut into chunks; small knob butter; pinch dried rosemary or thyme; 450g potato (1 very large baking potato is perfect), chopped into thick slices; 140g melting cheese, such as cheddar, cut into small chunks; 500g pack shortcrust pastry; 1 egg, beaten