Be sure to let the meat rest before slicing into pieces; allowing it to sit helps to retain the juices and flavor.
Ingredients
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Extra-virgin olive oil
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1 ½ pound boneless sirloin steak
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Coarse salt
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1 teaspoon ground black pepper
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1 onion, cut in half and sliced
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1 tablespoon unsalted butter
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⅓ cup red wine vinegar
Directions
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Preheat oven to broil.
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Place 12-inch cast-iron skillet under broiler for 2 minutes.
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Brush olive oil onto steak. Season both sides of steak generously with salt and freshly cracked black pepper.
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Add the steak to the heated pan and broil for 8 1/2 minutes.
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Transfer steak to a plate to rest. Heat steak pan on the stovetop over medium heat.
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Add onions and butter. Stir and cook until browned, about 6 minutes. Pour in vinegar and stir to scrape up browned bits on bottom of the pan. Let vinegar simmer until reduced, just about 1 minute.
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Pour any steak juices from the resting steak into the pan. Transfer the steak to a cutting board and slice into 1/4-inch slices. Lay the steak slices on a platter and pour the sauce over top and serve.