Heat your pan over a medium high heat and add some oil.
Sear off the steak chunks in batches, browning nicely all over. Season them with salt and pepper as you sear and set them aside.
Put some more oil in the pan and add the mustard seeds. Fry until they start to pop.
Julienne your onions and add them to the pot. Fry for 3 minutes or so until they start to brown and have lifted most of the fond from the pan.
Peel and chop your carrots into 1cm chunks and add them in. Fry off for another 2 minutes or so until they are startin to soften.
Mince or grate your garlic and finely chop the rosemary. Add it to the pot and cook until fragrant.
Wash and quarter the mushrooms and add them in. Cook them for 4-5 minutes until they soften and brown.
Add in the tomato puree and mix well. Once mixed, add in the flour and mix too.
Pour in the ale and mix to combine. Cook this off until the liquid has mostly gone and it has thickened up, about 5 to 6 minutes.
Pour in the beef stock and add the Worcestershire sauce and bay leaves. Cover and reduce the heat to medium low. Simmer for an hour covered.
Remove the lid and raise the heat to medium until it is simmering. Cook for another hour or so to reduce, stirring regularly to prevent it from burning to the pan. You want the liquid to have reduced by at least half and the remaining stew to be quite thick. Remove from the heat and allow it to cool.
Heat you oven to 180°C. Roll 3/4 of your pastry out and line the tin. Line the pastry with baking paper and add in some baking beans. Add into the oven and par bake for 10 minutes.
Remove the beans, wash with the egg and bake for another 5 minutes.
Spoon the cooled filling into the pie shell. Roll out the last of the pastry, egg wash the edges of the pie shell and lay the pastry over the top. Crimp the edges with a fork to seal. Cut a couple of small holes in the top of the pastry lid to create some steam vents and put it into the oven.
Bake for around 30 minutes, until the pastry is golden and crisp. Remove from the oven and leave for 10-15 minutes to cool and settle. Slice and serve!
