Corned beef and onion pie

Name: Corned beef and onion pie
Ingredients: 15g/½oz butter; 1 tbsp sunflower oil; 1 large onion, sliced or chopped; 2 celery sticks, trimmed, peeled to remove stringy bits, sliced; 2 medium carrots, cut into 1cm/½in dice; 300g/10½oz potatoes, preferably Maris Pipers, peeled and cut into 1cm/½in dice; 2 tbsp tomato ketchup; 1 x 340g/11¾oz can corned beef; freshly ground black pepper; 300g/10½oz plain flour, plus extra for rolling; good pinch sea salt; 175g/6oz cold butter, cubed; 1 free-range egg