Preheat oven to 180°C.
Lightly dust the diced beef with flour.
Heat a large heavy based fry pan to medium heat, add a good drizzle of olive oil and brown the beef in batches to sear the outside. Remove from the fry pan and set aside.
Add the onion and garlic to the same fry pan, without cleaning it. Add more oil if needed. Sauté until the onions become transparent.
Add the sliced celery, carrot and mushrooms and sauté for a further 5 minutes.
Add the tomato paste and stir through for a few minutes before adding the beef stock, rosemary and bay leaves.
Add the seared beef back into the pan and stir through. Check the seasoning and add salt and pepper as needed.
Spoon out the pie mixture along with any liquid into a large ceramic pie or casserole dish.
If your pie dish is rectangle shaped, lay the square pastry sheet flat over the top and add the extra 1/2 pastry sheet overlapping at one end. Gently press to join the two sheets.
Cut off any excess pastry around the edges. Using your thumb and index finger gently press along the edge of the pastry creating a pattern. Slice two small holes in the middle for hot air to escape while the pie cooks.
Using a pastry brush, brush over the pastry with the egg yolk wash.
Place in the oven and bake for 40-45 minutes or until the pastry is a golden brown colour.
Remove from the oven and allow to cool for a few minutes before serving.