INGREDIENTS
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3/4 pound flat iron steak
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1 medium onion, sliced
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2 teaspoons butter
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1/3 cup plus two teaspoons olive oil
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2 tablespoons red wine vinegar
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1 tablespoon Dijon mustard
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1/2 teaspoon garlic powder
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2 teaspoon Worcestershire sauce
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1/4 teaspoon kosher salt (plus more for seasoning steak)
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1/4 teaspoon pepper (plus more for seasoning steak)
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2.5 ounces baby romaine leaves
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2.5 ounces baby spinach leaves
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1 cup thinly sliced crimini mushrooms