Prepare a grill or a grill pan for medium-high heat. Lightly oil the grill grates. Remove the steak from the refrigerator and let it sit out while you heat the grill, 20 minutes.
Toss together the parsley, oil, lemon juice, crushed red pepper flakes, garlic and salt and pepper to taste in a small bowl. Set aside. (You can make this up to 2 hours in advance.)
Season the steak on both sides generously with salt and pepper. Grill until the meat is charred and medium-rare inside, about 2 minutes per side. Transfer to a cutting board and let rest without touching the steak for 5 minutes (don't rush this--the secret is keeping the juicy flavor inside the steak). Slice the steak across the grain, transfer to a serving platter and scatter the chimichurri all over the top.