Make the chimichurri sauce: combine all ingredients up to olive oil. Stir well. Slowly whisk in the olive oil. Salt and pepper to taste. Set aside.
Take flank steak out of the refrigerator and season with salt and pepper and brush with a little olive oil. Set aside. Preheat an outdoor grill or indoor grill pan to medium high. Grill the meat until it is medium-rare, about 5-7 minutes per side, depending on the size of the steak.
Brush onion and corn with olive oil and place on grill next to meat. Grill until corn and onion are lightly charred. Remove from heat and set aside.
When meat is done, place on cutting board and brush with 1 T of the chimichurri sauce. Let rest for 5 minutes before slicing. Slice into thin slices, cutting across the grain.
Slice the onions and cut the corn off from the cob. Place arugula or your favorite