To make pastry; Place flour, KEEN’S Mustard Powder, butter, cheese and salt into a food processor and process until combined. Add egg and enough water, 1 tablespoon at a time, to bring the mixture together.
Tip mixture onto a large sheet of baking paper and knead briefly forming a smooth dough. Divide dough in half, roll out both halves between two sheets of baking paper to approximately 2 x (30cm x 30cm) 0.5cm thickness sheets. Use one pastry sheet to line a lightly greased standard pie dish, leaving some pastry hanging over the edges. Cover and set aside. Reserve second sheet for pie lid.
To make pie filling; Heat oil over high heat in a medium sized heatproof casserole dish. Add onion and cook for 3-5 minutes until softened. Place flour and mustard powder into a freezer bag add the beef and toss to coat.
Add beef to onions and brown, add tomato paste and cook for 2 minutes. Add remaining filling ingredients, except mushrooms. Season with McCormick Sea Salt and Black Pepper, stir well, then cover reduce heat to low and simmer or 2 hours, stirring occasionally.
Add mushrooms and cook uncovered for 10 minutes or until the liquid has reduced to a thick gravy. Set aside and allow filling to cool. Preheat oven to 200ºC (180°C fan forced).
When pie filling has cooled, spoon mixture into prepared pastry lined pie dish and lay second pastry sheet on top, trim the overhanging edges and crimp to seal. Cut a small air hole in the top of the pie.
Place pie on a baking tray, and bake for 60 minutes until the pastry is golden brown and the filling is hot.
If short on time; divide and conquer! Make pastry and filling one day then roll out and assemble the next.