In a large bowl, combine the flour, black pepper, garlic powder, onion powder, and salt. Lightly dredge each piece of beef in the flour mixture. Shake off excess flour. Set aside pieces until all are dredged.
Preheat oven to 160C. In a Dutch oven, or ovenproof saucepan over medium heat add 1 Tablespoon olive oil. Brown the beef on all sides. Do in batches so the pan is not overcrowded. Add another 1-2 Tablespoons of oil as needed to brown the beef.
Put last Tablespoon of olive oil into the pan. Add in the onions and garlic. Saute for about 2-3 minutes to soften onions.
Add in the Guinness Stout and use a wooden spoon to pick up all the brown bits in the bottom of pot. Add in the mushrooms, carrots, celery, beef stock, and bouquet garni. Season with salt and pepper, to taste. Place cover on pot and place in the oven. Center rack is best.
Let braise for about 2 1/2 hours. Take about 1/4 cup of the Guinness/broth liquid and place in a cup. Stir in 1 teaspoon corn flour, combine well. Add back into the beef mixture.
Check the beef, should be tender. Add another half hour of cooking, if needed. Remove Bouquet Garni and discard. Check seasoning, adjust if needed.
If using a 25 - 30cm pie dish. Put the beef mixture into the pie pan. Lay puff pastry sheet on top of the pie. Make a few slits in the pastry to release stem. Brush with the egg wash. Bake on middle rack for about 15-20 minutes. Check at 15 minutes. Pastry should be golden. Serve immediately.