To make the marinade: Combine ½ cup chopped fresh rosemary, garlic, 1 cup olive oil, 1/4 cup lemon juice, black pepper, ½ teaspoon red pepper flakes and ½ teaspoon salt in a blender or food processor. Blend until smooth and set the marinade aside.
Pierce the meat with a fork a few times and then place the flank steak in a shallow baking dish. Pour the marinade over the steak and flip to coat both sides. Cover and refrigerate for 4 to 6 hours.
To make the dressing: Combine in a jar, 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. Cover and shake the dressing to combine. Shake again before using.
Preheat an outdoor grill for medium-high heat and lightly oil the grate. Remove the steak and discard the used marinade. Season steak with salt and black pepper. Place the steak on the pre-heated 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 a rosemary sprig to baste the steak while it cooks. An instant-read thermometer inserted into the center of a steak should read 125°F for rare. 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.