Beef Stroganoff Sauce with Meatballs
Ingredients:
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½ teaspoon vegetable oil, or as needed
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1 pound ground sirloin
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3 tablespoons Worcestershire sauce
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1 egg
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⅓ cup dry bread crumbs
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½ small onion, chopped
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salt and ground black pepper to taste
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¼ cup butter, divided
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1 (16 ounce) package mushrooms, sliced
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2 tablespoons all-purpose flour
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1 (10.5 ounce) can beef broth
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1 teaspoon ground mustard
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⅓ cup sour cream