In a large saucepan, cook the sliced onions, olive oil, sugar, 1 tsp. of salt and 1 tsp. of pepper over medium-high heat until the onions are caramelized. This should take about 15 minutes, stirring occasionally. Meanwhile, mince the garlic cloves, and place them into a medium bowl. After the onions are caramelized, scoop them into the bowl with the garlic. Set aside.
Place your saucepan back on the stove, and return to medium-high heat. Cut the steak into bite-size cubes, and toss with the 2 tsp. salt, 1 tsp. pepper, and 2 tsp. garlic powder. Brown in your saucepan until the edges have reached a deep brown, and the meat is cooked through. Remove the steak from the pan, and place into a small bowl, set aside.
Preheat your oven to 400 degrees. Line your baking dish with the bottom pie crust, and prick with a fork. Bake for 15 minutes, until it's reached a light golden brown. Set aside.
Back on medium-high heat, add the sliced mushrooms, 1 tsp. salt, 1 tsp. pepper, and beer into your pot. Cook until the beer has reduced significantly, and then add the onions and garlic back into the pan. Stir in the tomato paste, and then the beef stock.
Add the cubed potatoes, 1 tbs. salt, 2 tsp. pepper, rosemary, thyme, and 1 tbs. garlic powder, and boil for about 10 minutes, or until the potatoes have cooked through. In a small bowl, stir together the cornstarch and water until combined. Pour into the boiling stew, and stir constantly until it becomes very thick. Stir in the cooked steak.
Pour the mixture into your prepared bottom crust in your baking dish. Place the top crust over the top, and slice vent holes. Beat the egg, and brush over the top of the crust. Sprinkle with salt and pepper lightly. Bake for 25-30 minutes, or until the top crust is golden brown. Serve hot.