Pat the flank steak dry with paper towels. Rub both sides with olive oil, salt, pepper, garlic powder, and smoked paprika. Let the steak rest at room temperature for 20 to 30 minutes.
Prepare the chimichurri sauce by combining parsley, garlic, oregano, red pepper flakes, red wine vinegar, olive oil, salt, and pepper in a bowl. Stir well and set aside for at least 15 minutes to allow the flavors to blend.
Preheat an outdoor grill or grill pan to medium-high heat.
Place the steak on the hot grill and cook for about 5 to 7 minutes per side for medium-rare, depending on thickness.
Transfer the steak to a cutting board and let it rest for 10 minutes before slicing.
Slice the steak thinly against the grain to ensure maximum tenderness.
Arrange the sliced steak on a serving platter and spoon the chimichurri sauce generously over the top.
Serve immediately with extra chimichurri sauce on the side.