To prepare the beef:
Preheat the oven to 325°F.
Heat a medium size casserole with oil over medium heat and sauté the bacon until crispy, transfer to a clean plate and set aside.
Season the flour well with salt and freshly ground black pepper. Toss the diced beef into the seasoned flour and dust the excess flour off.
Add a little more oil to the casserole and fry the beef in batches until browned on all sides.
Place the seared beef pieces on the clean plate with the bacon and set aside.
Add a little more oil to the casserole and gently sauté the onion until just softened but not brown. Deglaze the pan with some stout.
Return the beef and the bacon and its juices to the casserole and stir in the diced butternut squash, tomatoes, tomato puree, sugar, mustard, bouquet garni and the remainder of the stout to the pan and season with salt and freshly ground black pepper. It's important that the meat is just barely covered with liquid, therefore add a little beef stock if required.
Bring to the boil, cover and place in the oven for about 2 1/2 hours or until the meat is tender and the sauce has thickened slightly. Stir from time to time, but take care not to break up the meat.
During the last 10 minutes of the cooking time, heat a medium frying pan with a little butter and season with salt and pepper, add the mushrooms and sauté for about 2 to 3 minutes. When the casserole is cooked, add the mushrooms and simmer for 3 to 4 minutes. Check the seasoning, adding salt and pepper if required.
To prepare the pastry:
During the cooking time, make the pastry. Sift the flour and salt into a large bowl.
Add the diced butter and rub with your fingertips until the mixture resembles fine breadcrumbs.
Add the baking powder and mix well. Stir in the mashed potatoes and pour in just enough cold water to form soft pastry dough. Turn the dough out on a floured surface and knead very lightly.
Place in the refrigerator to chill for about 230 minutes.
When you are ready to make the pie, remove the bouquet garni from the beef.
Carefully roll out on a floured surface, the pastry shouldn't be too thin and roll out the shape to fit the casserole dish (or the pie dish if you have transferred the beef and Guinness filling into one).
Increase the oven temperature to 430°F.
Using the rolling pin, carefully lift the pastry over the casserole and "tuck" the pastry into the sides leaving a narrow edge. Using a sharp knife cut the excess pastry off and crimp the edge. Shape the excess pastry and place on top (see tip below).
Brush with egg wash and transfer in the oven for about 20 to 25 minutes or until golden and baked.
Remove from the oven and leave to "rest" for about 5 to 6 minutes before serving with champ.
Tip: To make a shamrock to decorate the top - use three hearts of pastry and place upside down.