Smash the garlic, sprinkle with 3/4 teaspoon salt and work into a coarse paste using the side of a large knife. Transfer to a small bowl and stir in the parsley, olive oil, lemon zest and juice, and pepper flakes.
Heat a large skillet over medium-high heat. Lightly rub the steak on both sides with oil and sprinkle generously with salt and pepper. Put the steaks in the skillet and cook, turning once, until well browned on the outside and medium-rare on the inside, 2 to 3 minutes per side. Transfer the steak to a cutting board and let it rest for 5 minutes.
Thinly slice the steaks against the grain and divide among 4 plates. Top with the gremolata and serve with rice or potatoes.
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