Roll your puff pastry sheet out and trim into a rectangle the exact dimensions of your baking dish. Decorate the edges with excess pastry cut into shapes.
In a medium sized bowl, add your steak, flour, onion granules, salt and pepper. Mix to combine.
In a large saute pan over a high heat, fry your steak in batched for 2-3 mins until tender and starting to caramelise. Remove from the pan.
Heat your butter over a medium heat in the same large saute pan. Add your onions to the pan and cook until soft, about 5 mins. Add your mushrooms and turn up the heat. Cook for about 3 mins, until the mushrooms are soft and the onions are starting to caramelise. Add your garlic and paprika, cook out for 1 min.
Add your beef stock and milk and bring to a simmer. Add your macaroni, pop a lid on and cook for 10 mins until tender, stirring occasionally to prevent sticking.
Add your steak, double cream, sour cream, cheddar, mozzarella, mustard, parsley and seasoning to the pan. Stir to combine.
Lay your mixture in a large baking dish and top with your pastry sheet. Brush with egg wash.
Bake for 25-30 mins until the pastry is risen and golden. Serve immediately!