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Combine 1/2 cup chopped fresh rosemary, garlic, 1/3 cup olive oil, 1/4 cup lemon juice, black pepper, 1/2 teaspoon red pepper flakes, and 1/2 teaspoon salt in a blender or food processor. Blend until smooth and set marinade aside.
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Pierce the steak with a fork 20 to 30 times. Place in a shallow baking dish. Pour marinade over steak and flip to coat both sides. Cover and refrigerate for 4 to 8 hours.
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Combine 1/3 cup olive oil, 3 tablespoons lemon juice, lemon zest, 1 teaspoon minced rosemary, 1/8 teaspoon red pepper flakes, and a pinch of salt in a jar. Cover and shake the dressing to combine. Shake again before using.
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Preheat an outdoor grill for medium-high heat and lightly oil the grate.
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Remove steak from marinade; scrape off any chunks of marinade. Discard used marinade. Season steak with salt and black pepper.
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Place steak on the preheated grill and cook until browned on the outside and red on the inside, about 6 to 8 minutes per side. Place 2 tablespoons of the olive oil-lemon juice dressing in a small bowl. Use rosemary sprig to baste the steak while it cooks. An instant-read thermometer inserted into the center of a steak should read 125 degrees F (52 degrees C) for rare.
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Remove steak to a platter and allow to rest about 6 minutes before slicing. Cut in half lengthwise, then into 1/4-inch-thick slices at an angle against the grain. Drizzle with additional dressing.