Chef John's Salisbury Steak
Ingredients:
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1 pound 85% lean ground beef
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½ teaspoon freshly ground black pepper
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1 teaspoon dry mustard
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2 tablespoons ketchup
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1 tablespoon Worcestershire sauce
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1 teaspoon soy sauce
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1 large egg, beaten
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1 pinch cayenne pepper, or to taste
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½ cup plain bread crumbs
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2 tablespoons salted butter
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12 large white button mushrooms, sliced
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1 cup diced onion
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1 pinch salt
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2 tablespoons all-purpose flour
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2 tablespoons ketchup
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1 tablespoon Worcestershire sauce
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3 cups high-quality, low-sodium beef broth
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salt to taste
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2 tablespoons salted butter