Heat oven to 350 degrees if you are not using an outside grill or grill pan.
Cover flank steak with salt and pepper on both sides. Rub all over the meat and then let it sit until it comes to room temperature.
Heat grill, grill pan, or fry pan. Brush on oil.
Once fully heated, sear the flank steak for 2-3 minutes on each side. If your flank steak is extra thick, you may need to add an extra minute to each side.
Pull steak off the grill and set in a glass pan. Pop it in the oven for 3 minutes or until the internal temperature reaches 130-135 degrees for medium rare. You can skip this step and just grill the steak for extra time on the grill but it may be more well done or tough.
Remove from oven and let sit 5 minutes. Slice the flank steak against the grain and drizzle chimichurri sauce on top.