Chef John's Salisbury Steak

Ingredients (16)

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