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1
Set your oven at 375 degrees.
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2
Toss the julienned onions with 2-3 teaspoons of olive oil. Spread them on a baking sheet and sprinkle with sugar. Roast the onions until soft and somewhat caramelized, about 20 minutes or so. You do have to stir them about once or twice during the cooking process. When they've colored nicely, set them aside to cool.
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3
Heat 20 ounces of water in a kettle and add the broth crystals and set liquid aside.
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4
Mix mustard powder and Worcestershire sauce together to make a sort of slurry.
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5
Put about two tablespoons of bacon fat (you can substitute olive oil if you need to) in a large saute pan. First brown your sausages whole until they've got some nice color on the outside. They don't need to be cooked through. Remove from pan and set aside to cool. When they are cooled, cut them on the diagonal, in two-inch pieces.
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6
In the same pan in which you cooked the sausages, place the potatoes cut side down and cook them until almost fork-tender and golden brown. Remove them from pan, sprinkle with salt and pepper and set aside to cool.
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7
In the same pan, saute the mushrooms until they are caramelized on one side, adding more fat if necessary. When caramelized, remove from pan and set aside to cool.
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8
Make certain there are about two tablespoons of fat in the hot pan and add two tablespoons of flour to make a roux. Cook the roux until it's fragrant - about 2-3 minutes. And then whisk in the broth. Add the mustard/Worcester sauce slurry. When the gravy is thick, slowly pour in the beer, taking care that it does not foam out of the pan. When the beer is settled and incorporated, add the roasted onions. Taste for seasoning at this point.
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9
Roll out the bottom crust and place it in a 9-inch deep, glass pie plate. Layer the sausage pieces, potatoes and mushrooms into the pie plate. Pour the gravy over them. Cover with a top crust. Brush with a salted egg wash.
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10
Place the pie on a low rack in the oven. Lower the temperature from 375 to 350 degrees, immediately upon closing the door. Cook the pie for 45 minutes. Then move the pie to a top rack and cook for another 10 minutes or a bit longer until the crust is golden.
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11
Cool the pie slightly 10 to 15 minutes before serving.
