Using a food processor, pulse the cilantro and garlic until finely chopped. Blend in the olive oil, vinegar, crushed red pepper and 3/4 teaspoon salt. Set aside.
Heat a grill or grill pan over medium heat. Pat the steak dry and season with salt and black pepper. Brush the onion slices with olive oil. Add the steak and onion to the grill, cover and cook for about 5 minutes on each side for medium-rare. Transfer the steak and onion to a cutting board and let stand. Grill the tomato slices for about 1 minute on each side.
Thickly slice the steak and serve alongside the onions and tomatoes with the reserved cilantro sauce on the side.