For the sauce: Place the cilantro, oil, red pepper, garlic, parsley and some black pepper in the bowl of a food processor
until the sauce has a chunky but smooth consistency.
For the sandwich: Preheat a grill
to medium heat. Slice the steak into 1/2-inch strips. Sprinkle the garlic and some salt and pepper over the steak. Saute the onions. Grill the steak to medium-rare. Mix the onions and steak together and top with the cheese
. Halve the bread horizontally and lightly toast. Place the meat, onions
and cheese on the bottom slice, then drizzle
with the chimichurri sauce
and top with the top half of the bread. Slice the sandwich diagonally into thirds.